Games would probably do more reading than writing, so it's unlikely that an SD card's performance would come into play (outside of initial download-to-SD or whathaveyou).
Granted, I've always had a class 6 since that was my largest, but recently went with a Class 2 32GB. I know that before and after the FPS lock (toast kernel, ~52 FPS), Caligo Chaser had a noticable improvement in smoothness and responsiveness. This was still on the Class 6 card. I haven't tried any OpenGL games since the unlock.
I can give it a shot later on to see if it makes a difference. If class would make a difference it'd be very dependent on the game, and only if the game did a lot of writing *during* gameplay. At the moment, I don't really see a net impact from a lower class card for gaming.