The FF equivalent of the 17-55mm f/2.8 is a hypothetical 27-88mm f/4.5 lens. So, compared to the 17-55 on APS-C, the 24-105 on FF is wider, longer, and the aperture is 1.3 stops wider in terms of DoF for the same framing. Yes, it's still f/4 for exposure, but the FF sensor also has at least 1.3 stops less ISO noise (more like 1.5 going from 40D to 5DII), so you can bump the ISO to make up the stop and still be ahead on noise. Overall IQ of the 24-105 on FF is better than the 17-55 on APS-C, and you keep IS. The only thing you're giving up is the activation of the f/2.8-sensitive center AF point.