With ultra-low frame rates, possibly 60 to 120 fps -- with much higher settings, you may see that drop to as low as 10 or 15 fps. It would depend on what game you're playing, what graphics settings you have the game set at, which graphics processor in the MacBook Pro you're using (the 9400M or the 9600M), what resolution you're playing the game at, the age of the game, etc.
There's too many factors to throw out a specific frame rate, as the frame rate will vary from game to game, from setting to setting, from resolution to resolution, from graphics chip to graphics chip, etc.
I would tend to say that you're not going to get supremely excellent frame rates out of a monitor that large. Would they be "playable?" Most definitely... will they be spectacular? Probably not. Beyond that, I really can't say. Too many factors to take into consideration.
