30lbs is more than enough for a drysuit, hell in a drysuit you need a smaller wing than in a wetsuit. A buoyancy compensation device primary task is to compensate for changes in buoyancy. That change in buoyancy is the mass of the gas in the tank that would cause a buoyancy shift, about 6lbs in the case of an AL80 up to about 10lbs for the larger steel tanks, and in the case of a wetsuit the compression at depth. A 3mm we say typically has about 6lbs of positive buoyancy, 7mm about 14, and a farmer john around 20lbs and it will lose about half that once you hit ~30ft, and will lose about 3/4 of it by the time you hit ~100ft due to compression of the closed cell gas pockets in the rubber. With a drysuit, we consider them to be constant buoyancy devices since you would be adding gas to compensate for that compression so the only thing the BCD has to do during a normal dive is compensate for the gas in the tank. Since that is relatively small many agencies/instructors advocate that you only use your drysuit for buoyancy control when diving with single tanks. Another can of worms not worth opening here as there are tons of threads about it on this forum.
The other things the BCD has to do is comfortably float your head at the surface, this is a point of contention with those that like 20lb and smaller wings, but I sink like a brick so I don't dive anything smaller than 30lbs so I can get my head out of the water. The only other thing is that it has to be able to float the rig at the surface without you in it, so if you use a weighted STA, attach weight pockets, etc. it has to be able to float all of that at the surface. If you use a weight belt then the lift is not an issue you have to worry about and even with a weighted STA a 30lb wing will handle that just fine.
40lb wings are excessive IMO and while you see them in the PNW area of the US with guys that dive in frigid water in wetsuits, if you're in a drysuit it is certainly excessive.
