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First: al80's are only .6lbs more negative than an al40 when full.... you shouldn't lose buoyancy from half a pound, especially on OC. If the bottle is not fully empty/super floaty, it should be very close to neutral which should negate any difference in passing it around.
Second, and arguably more importantly, who says that you have to use an al80 for trimix diving and why you can't for nitrox diving? The last cave trip, if you follow the need for 1.5x gas for deco, I needed an AL80 for decompression and I was only diving to 100ft. Granted it was for 3 hours, but an AL40 is not large enough for what I was doing.
Third: said dives required the use of 2 al80's for bottom stage bottles, and the al80 of O2 for decompression. 3 bottles to move around, and since I was in a cave that has a restriction to the entrance after the deco room, the bottles had to stay in place. From there, they get moved to my butt rails and chill out, but all of the bottle handling is there.
Had that been conducted like a "full trimix dive" or actually been one in a similar profile cave, one of those bottles would/could have been a bottom stage, another would have been my travel gas/deep deco, so either EAN32, 35/25, 50/50 or whatever you choose, and the other would have been O2. Bottom stage handling doesn't change, and you drop off O2 in the deco room, travel mix could have been EAN40 because of the cave profile so would have been dropped off at the tie in point at 80 ish ft. Return of dive would have you pick up and verify etc etc the deep deco mix and switch off of back gas for the first deco stops. Up through the restriction and into the deco room, you conduct a normal gas switch procedure onto backgas *assuming bottom mix is not hypoxic at that depth* and done a gas switch to the O2 bottle. If diving a wicked hypoxic mix like 10/70 or something, you may want to leave the travel gas up front for air breaks instead of switching over to 10/70 because of ICD and the fact that it's borderline for PO2 though I'm not sure how GUE teaches that since they want all bottles on the left, which are the things you want an instructor for.
The dive itself wouldn't really have been any different from a bottle switching standpoint, the only difference would have been the use of a deep deco bottle/travel gas.
So it really all depends on the dive profiles you are currently doing and the ones you want to do. For me to go from 3+hr long nitrox dives at 100ft with multiple stages and then go over to full trimix, there are no new skills. I'm already doing multiple stage handling, so the only difference is the addition of He to the gas planning and dive planning *from both a physiological and logistics standpoint*.
If however you are coming from a training background where the only "extra" bottle you are carrying is a 30 or 40 of O2, then yeah, the learning curve is going to be steep because bottle handling isn't something you are familiar with, especially in OW where you aren't staging that deco bottle and aren't used to moving it around.
some good videos from ISE on how they teach multi stage handling.
