We're talking about displacing about 4 pounds of water distributed over the area of a human body...Think about how much your body changes shape when you add or lose 4 pounds...If you are wearing an undergarment under the shell, it will easily absorb the delta without becoming a stability or flow problem...
Except that when you gain or lose 4 pounds of weight as fat, it is evenly distributed. Not so while gaining air in a drysuit. The air preferentially collects around your shoulders. A weight gain of 4 pounds at the shoulders is obvious. This becomes even more obvious when using a larger tank. Remember that the aluminum 80 was used for this example, so larger tanks can have a much larger impact on the amount of extra air here.
A noticeable difference upon getting your weight right will be that you will not be using the BC during the course of your dive. This is more of a side effect, than a goal or personal preference.
This has very little to do with your weighting, and everything to do with your choice to add air only to the suit. You can be significantly overweighted and still choose to use the drysuit only. It doesn't make it the best method - it just remains a possibility.
Unless you are too heavy to begin with and already have a bunch of excess air in the suit, that half gallon isn't going to make a big difference, and it will go away as the dive progresses. Plus it isn't really necessary to have it in there in the first place as respiration will also impact what you need in the suit. Realistically, it will likely be less than a quart change in suit volume over the course of a dive. That's fast approaching negligible.
At this point, you are starting to do a lot of hand waving to try and minimize things.
I'd like to address your comment that the lungs will take care of all of this. The average vital capacity of an adult is between 3-5 liters. That's the amount of air that an individual can blow out after fully inhaling. With normal breathing, individuals are not fully exhaling with each breath. People have what's called an expiratory reserve volume (ERV), which is the amount of air you can force out of your lungs after a normal respiratory exhalation. This is typically about 1/4 of your vital capacity. That means that the average person can only increase their lung volume by an additional 2.25-3.75 liters. Of course, that means that the persons lungs would be completely full at that point. When people DO breathe, lung volumes typically shift by 500mL. So leaving room to breathe at the maximal inhalation, we're now talking about a capacity of increasing the average persons lung volumes by 1.75-3.25 liters. This means that they would be breathing in and out, but at the end of breathing out, they would have completely full lungs - not a real fun way to spend any significant amount of time.
You are talking about offsetting a weight of roughly 2kg, which is 2 liters of air. For the average adult, this is more than possible for the individual that only has an additional 1.75 liters to work with, and over 60% of the reserve of people near the upper end of average. The "quart" of air you randomly made up in your example is about a liter, which is more reasonable, but still a significant amount, as it accounts for 31-57% of the average adults available lung capacity. NOT an insignificant amount.
This is exactly why people tend to put air in the suit or the BC - it's not really comfortable to shift lung volumes by that amount, even if it IS possible. Of course, if you start talking about larger, or even double tanks, this quickly shifts into the realm of impossible to manage with lung volumes alone.
There's a reason it is called a standard 80. It's the standard tank that most people dive everywhere I have scuba dove.
Yeah, it's called a "standard" 80 to differentiate it from the "neutral 80" made by Catalina. The only thing that makes aluminum 80s so common is that they are cheap as compared to steel tanks, so most rental fleets are composed of the cheaper tanks. Since that is what most people see when they get started in diving, many people gravitate towards aluminum tanks when they buy their own gear, because that is what they have had experience with. Many people who do a lot of diving, and certainly people who are doing more advanced dives, know that the aluminum 80 isn't the best tank for their purposes, and start buying steels.
For comparison, a typical DS will have upwards of 3 gallons of air in it, just to keep the squeeze off. Obviously, this will depend on the size of the diver. Varying that by a quart or even a half gallon isn't going to be very noticeable. It's going to be an 8% to 16% variation.
OK, I'll run with the numbers you made up for this example. There is air in a dry suit and inherent buoyancy in drysuit undergarments, even when the suit is squeezed. This dead space is evenly distributed over the body of the diver, and there really isn't much you can do about it. That's why drysuits are just plain more floaty than wetsuits. Of course, when you start adding air, that air tends to not distribute evenly. Now even if it did, 8-16% variation may still be noticeable, since it is increasing the diameter of the diver profile. It's even more noticeable when that extra 8-16% is localized around the shoulders, at the front part of the diver, where it would cause the most resistance to forward motion.
Again, I'll say, If you need buoyancy, particularly if you need a lot of buoyancy, use the BC to get it. If that is the case and you are not a tech diver, there is a good chance you are diving overweighted. If that's the case and you fix that problem, you will likely find that you won't have use for the BC during the dive. In this case, getting it right has a larger than usual payoff. All the usual benefits are gained, plus you find that you have one less unstable air bladder to deal with, so you can spend more time enjoying your dive and less time fiddling with you gear.
All you are managing to point out with all of this is that you are rather limited in your scuba diving experience. You continue to beat a dead horse, and oddly, that horse shouldn't even be in the race here.
Even overweighted divers can use their suit only for buoyancy. Even tech divers, while carrying multiple tanks and stages, are able to do this - and we are profoundly overweighted in this configuration. In fact, it's kind of the point - drysuits are a wonderful source of redundant buoyancy.
ALL properly weighted divers will have need for extra air, either in the BC or in the suit, to offset the weight of their breathing gasses. Even extending into tech, you can shovel that gas into your suit and stay afloat in most cases.
The point is that even though you CAN do it this way, it doesn't mean it's the best way.
Now to address your point about the "unstable air bladder" you mentioned. The drysuit is much less stable of a place to put air than your BC. The drysuit allows the air to shift a lot more than a BC does. The drysuit has a much larger capacity to allow for air expansion than the BC does. The variable expansion of air in a drysuit has a much larger ability to upset your horizontal trim than it does in the BC. All of this becomes very obvious with changes in depth.
Your last comment, though, about having more time to enjoy your dive and less time fiddling with your gear takes the cake, though. Are suggesting that managing a BC is too complicated for a diver? Try lifting up your inflator hose to vent the BC while having your drysuit dump valve open. See what happens? Both vent at the same time - it was designed to work that way. What makes this even more funny, though, is that this problem you are trying to create is the same problem you try and minimize elsewhere in your argument. You have gone on and on that "there is only like a quart of air needed." OK...fine...you can't manage a quart of air in your BC as opposed to your suit? A liter?
Honestly, I really don't understand your resistance to all of this information. I figured with all of your posting about drag and the limitations you perceive it imposes on diving, would have made you the poster child for wanting to try this out for yourself. The thing that gets me the most, however, is that this information was presented as something to consider for someone that may end up advancing in their diving at some point. You are stoutly presenting the suit only method as the best way for everyone, even though there may be some downsides to doing that for divers that choose to advance further in their diving than you. After repeatedly beating the "overweighted" horse, maybe it's you that needs to "lighten up."
