No, it's not a good idea. Most fisheyes use an equisolid or equirectangular projection, not a rectilinear projection like an ultrawide will. There is more distortion, and while that distortion might be more extreme at the edges, it is still different from the typical barrel distortion of an ultrawide zoom lens.
In addition to this, the Canon EF 8-15mm f/4L fisheye lens is not a diagonal fisheye at the 8mm end of the zoom range; it's a circular fisheye; it's only diagonal at the 15mm end (see: the-digital-picture.com review of 8-15L). What this means is that the entire full-frame sensor is not covered by the image circle of the lens, instead, the entire lens's image circle fits inside the frame. When used on APS-C, this means that you'll get black vignetting at the corners: the image circle will still be too small to cover APS-C, either. You will only completely cover an APS-C sensor corner-to-corner if you zoom in past 10mm.
I am not really satisfied by the EF-S lenses available cause of missing small f-numbers, weather sealing etc.
Have you looked at the Tokina 11-16/2.8 DX II? It's not weather-sealed, but it is f/2.8 and does have a rubber gasket, and is designed for crop.