joule creep is definitely something people should be aware of.
400 fps with a .2 is roughly 1.48J... And with Xcortechs being able to work out the J rating if you properly set the BB weight, there shouldn't be any excuses not to be chronoing the provided .2s and the ammo you wish to run in the gun for the game.
Hell, one of the vendors in the states I order from sent me a bunch of business cards with a basic J conversion chart on it much like the one chaz pointed out.
You won't see much creep on an AEG.. maybe .1 or .2 of a J but you'll see massive swings on anything with expanding gas powered, gbbr, pistols, polar stars and daytona guns.
I futz with V2s, V3s and V6s. I could be wrong... but probably, most likely not, as far as I know.