I played Real Racing 3 on my 5th gen iPod Touch. Almost cut my thumb on the jaggies. Runs at about 20fps too. Brilliant.
This. I have a 4S and it ran sub-15fps (at best) and couldn't find an option to turn the detail down at all. I managed to have a poke around (jailbroken) and found some files that *might* let me use the 4 assets/config (fewer effects, I'd assume) on the 4S and buy me some performance.
Also found some config files that referred (by their filename) to Android... so yay?
But yeah, it may look nice and I'm sure it'd play nice if it had a decent framerate... but the framerate is balls. Moreso when you are tailing other cars and it slows down further.
Edit: Well, as it turns out... I'm a genius of some kind.
1) Navigate to the game folder via iFunBox or whatever (I used iFile, but I'm jailbroken... so I don't know if iFunBox will let you into the appropriate area if you aren't)
2) Browse through to ROW_Distribution_v1.0.1_H49.app\res\eds
3) Note the plist files. Each corresponds to an iDevice model. The 4S is commonly identified as iPhone4.,1, and it's safe to assume (I didn't look it up) that the 4 is the iPhone3,3. There are config files conforming to this, so I just renamed the 4.1 (4S file) file, made a duplicate of the iPhone3,3.plist (4) file and renamed it to iPhone4,1.plist.
Normally you can edit .plist files and sometimes tinker with individual settings, but not in this case. But thankfully it doesn't crack the sads after renaming around a bit and it now seems to run as smooth as it'll ever get... there's zero slowdown and it seems to run at a locked 30fps. And from what I can recall pre-tinker, it doesn't look all that different. In the least, it doesn't look ugly.
I can't vouch for doing this other ways where there's a difference in the screen resolution (ie. 5 to 4S, 4 to 3GS, iPad 3 to 2) so try at your own risk, but I don't think there's any really bad stuff that can come from it. Worst case, just redownload the game (though renaming the files back should do the trick).