I don't think it makes any difference. Any emulator and any ROM will probably fit into the ram, so you just need one read of the complete file (and that's what a lot of emulators do). For PSX ISOs this is not possible, but you don't need a lot of reading speed for that. The real bottleneck on performance is CPU/GPU power, not the sdcard class.
I'm not sure about Android games, but my guess is the same, if a game is not playable you won't fix it buying a faster SDCard.