I have a 10d and am looking to upgrade. One thing that I shoot a lot of is racing/sports. The auto focus has had me scream a few profanities every once in awhile and when lighting fades the noise can get real bad real quick. ISO 800 is as high as I will go.
I never had an issue with the focusing on the 10D, but I only used the center point. I shot at 1600 all the time and could get clean images, providing there was enough light.
As far as image quality under iso 800 you wont see any improvement if you stay with a crop , my 10D is on par with my EOS M. It actually handles the colors better, especially the reds.