New Digital Photo Prof does digital lens optimazation on it (Re: Will a 15-85mm...)
The just released new version of Canon's free Digital Photo Professional will do Digital Lens Optimizing to correct for loss in image quality caused by diffraction or lens aberrations, and sharpens the image to compensate for any loss of resolving power. At present it does this for about 25 Canon lenses and the EF-S 15-85mm is one of them.
Article demonstrating it.
I think you will find it wide enough. On the occasion it is not, that's why I have a tripod with a panning clamp on the head, a $15 Ebay focusing rail, and an L-bracket I made with steel corner braces from Home Depot, some bolts, T-nuts, a hex head bolt handle, and some scrap wood as a nodal rail for single row panoramas. Using DPP's lens corrections also helps the photomerge.