so the answer to your original question is "it depends". For AL80 doubles, you need very little lift because the gas volume is very small. In comparison, sidemounting say steel 19's *LP121's* in the US, needs 2x as much lift. One thing to note that is a pet peeve of mine, and hopefully this is a translation error, but you shouldn't be using your drysuit for lifting volume. It is not a buoyancy compensation device, it is exposure protection. Some people choose to use it as a buoyancy compensation device and in that case, yes it does have a lot of potential lift volume, but there are risks of doing that, especially when diving with multiple tanks and I urge you to read the pros and cons of using your drysuit for buoyancy compensation.
Doubles wings are actually very difficult to make in under 40lb lift versions because you have to make the wing wide enough to accommodate 8" doubles, and then give it enough room to wrap a little bit around the tank so you have room for canister lights, drysuit inflation bottles etc. To make the wing long enough to put the lift where you need it and wide enough, nominal 40lb is almost a minimum that the can be made.
In sidemount, we are trying to keep it as flat as possible and largely within the confines of a normal sized persons back. This dimensional constraint makes it extraordinarily difficult to make effective wings that are larger than 25-30lbs of lift. The current Razor 2.0 BAT wing claims 45lbs of lift, but it is horrible when inflated and turns into a beach ball on your back. The original Razor wing was basically an MSR bag and looked good but was very small. To get around this issue, as well as to be able to practice stupid things like switching bottles, many instructors are limiting sidemount use to al80's instead of steel bottles. Pros and cons.
Depending on how you decide to dive, you may or may not need to allocate much of any wing lift for stage al80's. If you are diving a balanced rig, which unfortunately where I dive, is completely impossible, and don't have large backgas tanks with large planned reserves *cave diving*, then you will have to add lead for every AL80 that you are diving with, and add about 1kg of lead for every stage you are planning to carry if you want to avoid having to jettison any al80 either in an emergency or if you are already planning on breathing your tanks down pretty low. In Florida where I do the vast majority of my cave diving, we are coming out with planned gas reserves of at least 8lbs worth of gas. That is enough to offset up to about 4 al80's when they start floating and if we breathe the stages to empty, and come out with our backgas right at 1/3 of their original pressure, then we are finally diving a balanced rig because of the positive buoyancy of the al80's.
We don't have the luxury of diving balanced rigs because the water is relatively warm compared to the waters of Europe *around 70F or 21C vs 75F/24C*, which is cold enough to require drysuits, but not cold enough to require them with lots of undergarments. Unlike Mexico, our caves are also comparably deep with many averaging around 100ft/30m depth *vs 50ft/15m* which further negates the ability to use wetsuits on the "big" cave dives do to buoyancy and insulation losses, but also means we are going through gas quite a bit faster than they do which requires the use of large steel tanks if we are going to do any meaningful penetration.
I'm currently diving a Hollis Katana and have been abusing it quite a bit for the last 6 months. It is perfectly fine with 2x Faber LP121's/19's, and 3x al80's in a trilam drysuit with only a Pinnacle Inferno for undergarments. It will also do the Fabers with a pair of AL80's in a 5mm wetsuit so long as the depth doesn't exceed about 30ft. At 100ft, the OPV was popping with just the Faber 121's and I was not able to achieve neutral buoyancy until the tanks had reached thirds *2400psi down from 3600psi*. For me, that means that 40lbs is about the minimum that I need for my style of diving, and would prefer to have 50lbs. My Dive Rite Nomad XT will come out if I need more lift, however it is a compromise due to having a higher profile due to the wing shape
