Fps is a pain on the iPad, so are most games that use the gyroscope.
I use these:
Have a Fling with your iPad
Fling is a joystick for iPad. It gives you controls you can touch for any game that has an on-screen joystick.
Fling's patent pending design provides auto-centering and force feedback. Once you start gaming with Fling, you'll never go back.
?Playing dual stick shooters using the Fling is, in a word, glorious.? - TouchArcade.com
?Fling doesn?t just improve your scores, it finally makes the iPad feel like a bona fide gaming device.? - AppleTell
I have a pair of these. Love 'em.