It takes it a few seconds to recover framerate after turning off AA.. exiting the game and going back in will probably help.
Maybe it is just me, but I can't get fraps to work in DX 11 mode, otherwise I'd take a benchmark.
edit: there's a newer version out than what I had.. I'll do a bunchmark run of one of the tracks and post the results here.
2x AA -
Frames, Time (ms), Min, Max, Avg
3007, 84062, 22, 40, 35.771
0x AA -
Frames, Time (ms), Min, Max, Avg
3277, 84936, 28, 43, 38.582
I think the minimums might be off because the second run was with no AA and had the textures loaded already.. it definitely gives a bit more performance, though.
edit 2:
4x AA -
Frames, Time (ms), Min, Max, Avg - First run
2721, 87554, 12, 38, 31.078
Frames, Time (ms), Min, Max, Avg - Second run
2553, 89849, 7, 39, 28.414
minimum framerate is where AA really kills, it looks like.. I've only got a core 2 duo @ 3.2Ghz, though.. the story may change with an i7 or similar cpu.
The sweet spot for me is 0x AA with everything maxed.. looks incredible (especially in replay mode) and driving too fast to notice any jaggies.
final edit: forced dx9 results -
0x AA -
Frames, Time (ms), Min, Max, Avg
4673, 90316, 43, 62, 51.741
2x AA -
Frames, Time (ms), Min, Max, Avg
4060, 83298, 41, 56, 48.741
4x AA -
Frames, Time (ms), Min, Max, Avg
4026, 84781, 40, 53, 47.487
Couldn't help myself.. replay screenshots from DX 11 mode everything maxed except AA, which is at 0x AA -
As you can see by the picture below, I haven't got the hang of driving yet..