Firstly, stop using .2s. You will get much better range and performance with a heavier BB. Hop applied will affect FPS downward.
Secondly, what you are referring to is Joule creep with a heavier BB. It will retain more of its energy (cos physics and stuff) and therefore maybe result in a creep up in power. Now, this is usually more of a GBBR problem than an AEG one but if you do change weight of BB be sure to rechrono your gun so that the new weight of ammo comes in under the Joules limit. This is why Joules are better than using FPS as its frankly too much of a ballache to try to figure out if my .3 is doing the correct FPS compared to a .2 out of the same rifle.
So, simply, put heavier ammo in, read the Joules score, if its under the site limit you are golden. If not, well then a test and adjust is needed.