In terms of CPU power, something like an Apple A6X clearly smokes it (not hard since Cell/Xenon were big pieces of crap - moreso than the Wii CPU in fact - a dirty industry secret that even Anandtech back in 2005 was privvy to).
In terms of GPU power though - there is still a generational gap or two between the latest iPads and the PS3/360.
Also just remembered a tiny bug with RetroArch on iOS. The screen does not stay awake while playing games.
I am not sure how you would be able to enforce that programmatically. For now, it will do just changing the option that controls screen dimming after a finite amount of time in Settings (iOS Springboard).