I had a play with one of these on a 5D3 the other day, and came to the conclusion that it's the same focal length at both ends.
What do I mean?
Well, I looked at the edges when it was at 8mm and in fish-eye mode, and then while at 15mm and the same area was covered i.e. a table was at the edge of the frame
Nice lens, but the fish-eye end just seems a bit of a gimmick.
Yes. It is pretty much the same degree fov. But that does not make them one and the same.
There is no "fish-eye end" since it is all technically fisheye across the whole range.
On a 5D3, you get a circular fisheye at 8 and a regular fisheye at 15. Two completely different things. Not a gimmick.
Lastly, the main motivation for this lens was to have one fisheye to rule them all with regard to providing a true fisheye with all sensors in one lens. This means all three of canon's sensors, if you happen to own them all have fisheye capability with it unlike any other fisheye on the market today (full frame, 1.3x, and 1.6x).