17-55 must be stopped down to at least f/4 to match the sharpness of the 15-85 wide open. It's not an f/8 lens really.
I'm not saying that the 15-85mm needs to be stopped down to perform adequately, I'm saying that even "wide open" (or as open as it can go), it doesn't have very shallow depth of field. At the tele end of its range, it is depth-of-field equivalent to f/8 or f/9 on a full frame, which is what motivated my reference to "f/8 and be there".
I agree that it performs pretty well at its maximum apertures. It's is a slow zoom, but as far as slow zooms go, it's pretty good.