If you plan to upgrade to 400FPS+, you may better also move to 0.25g BBs. 0.20g BBs are way too light at this power level and will not deliver the accuracy you need. If you plan to go over 450FPS+, consider the use of 0.30g BBs for maximum flight path stability.
By the way, heavy BBs do drag down FPS quite a bit (due to the heavier weight) but may produce higher impact (also due to the heavier weight). They are VERY expensive though… you should really take this cost element into consideration when defining your desired upgrade level.
Heavier BBs may produce higher impact. What this indicates is that FPS rating alone may not truly reflect how powerful an AEG is. While FPS measurement (feet per second) looks primarily at the “speed” of the flying bullet (i.e. the muzzle velocity)(!), Joule measurement (a system commonly in use in the UK) does take bullet weight into the equation to more accurately measure the force generated (i.e. the impact energy). In fact, we at The AirsoftPRESS prefer Joule measurement over FPS. However, it seems like the market in North America is biased totally towards the FPS system.