Hi cheese!
I've got the latest iMac too but went for the i5 and 2GB vram.
I've tested everything (I mean it).
In short, the res on the iMac is what's stopping you from getting hi fps on hi settings. Problem is that if you go with a lower res then, in my eyes, it looks crap.
Not like you can buy a new gcard and swap. Forget that route. You're stuck with what you have. Only upping available is RAM and that doesn't do anything to fps (I went from 4 to 12 without a single frame gained).
But don't be put down. You can make this game look gorgeous. You just have some tweaking to do...and some waiting for the promised ">50% more fps" patch due any year now.
I'll get back with my settings and other tweaks later. Got a kid that needs attention.
Btw, I soft overclocked my card but that's just for seniors. The iMac is fast to overheat.
I'll tell you more later.