Yeah, I have definitely seen better graphics and gaming performance.
I like to play Minecraft now and again, and on Mountain Lion there are times when the frame rate drops so low it becomes unplayable.
Granted I've only got an Intel HD 4000, and I crank all the setting to high and play in full HD... But, it has to be said, on Mavericks, those same parts just suffer no detectable frame rate drops,. in terms of game play anyway.