I'd suggest doing a thorough focus test with your Sigma 17-50 2.8.
I got one several weeks ago for my 7D and found that it would not focus properly using the outer focus points while in the middle of the zoom range. I tried another copy at a camera store and it did the same thing. Another member of this forum reported the same thing as well.
It's a great lens if it will play nice with your camera.
I did my homework before selecting the lens and have been thoroughly testing the lens in various scenarios (low light, with and without flash, different AF points, different AF modes). So far all appears to be working well. Thanks for the heads up though!!!