Same performance on battery and being plugged in AFAIK.
On my new SR MBP, game performance drops off a cliff in Boot Camp (WinXP) if running on battery only. I think this is due to NVidia PowerMizer technology scaling back the GPU speeds on battery-only, to save battery life. It's about half the speed as when it's plugged in.
I haven't tried to change/disable the NVidia powersaving tech to get equal performance, but there's almost certainly a way to do so.