For anyone who's been having controller problems with this, they are working on an update of controller support in their porting library and will be updating this game to correct the issue of the cars constantly veering to one side. On my system it had worked fine previously but some change screwed things up somewhere along the way. I think the current version is 1.07 on the app store. I contacted their support and was impressed to get a reply within a couple of hours.
The fixed new version is supposed to be handed off to Apple in about a week's time or so as of this writing. Then there is the time it will take for Apple to actually put it on the store so we can update.
It is a really great game. I used to play it on PC and PS2 and it will be nice to have it on my iMac. By the way, I am using a Logitech F-510 Rumblepad which works well for everything else on the system given it can do either Xbox or analog modes.
Hope this is helpful to some other Flatout 2 fan.