IR with unmodified Canon 40D and Hoya R72 filter.

I wouldn't recommend putting a strong filter on the front of a fisheye because you'll get vignetting. Not because the edge of the filter is visible, but because more oblique angles of light get darkened more. I used to use the 10-22 and with a 10-stop ND the vignetting was unusable at the wide end.

I have the same fisheye and I've been trying to get hold of a Kodak gel filter and tape it to the back of the lens. Some wide-angles have a slot for gel filters on the rear and I think this would work better because the light is at less of an angle.