As stated previously in multiple threads, the only thing a heavier BB does is shorten your range, might get some accuracy out of it under the right circumstances unless you are changing your spring. So when you have a spring shooting .20 at 400FPS then you will be shooting .25 @376 or around there. The BB does not change the spring, the spring is just pushing a heavier object. When you go to a .40 BB then your FPS will be even lower, and shooing a shorter distance.
The idea of snipers with heavier BBs is to trade off range for more accurate shots. f you go to an actual game with regulations you will see the real performance. Snipers generally are allowed 500FPS because they are a single shot. This will give them more range, but then a heavier BB is used limiting the range, but hopefully some added accuracy.
As Vince said, you will feel no difference between a .20 or a .40 with the same spring. Remember, the spring has to be measured at 500 FPS for snipers and 400 FPS for auto with .20.