When I've installed ROMs, I usually found performance to go up in regards to games. However, I've lately found that in stock, if I reduce the window animation to nothing, as well as the window transitions to nothing (from 1x or .5x), I've found performance has increased a lot. Not sure why that would affect a game, but games like Temple Run I've had running pretty choppy under stock until I turned off animation and transition.
Battery life seems about the same, maybe a little worse, but not by much.
I'm the type that likes to experiment and tweak, so a lot of times, I would install custom ROMs to play around with. I then find myself having a "flash" attack, wanting to try something new.
Luckily, with Safestrap v3, this makes it a LOT easier now!