Very, very impressed by the Canon 8-15

By its nature, a fish-eye is a specialty effects lens - no avoiding it.  Same as a tilt-and-shift or soft focus lens.  By definition and design it features a specific, repeatable optical distortion, which is an effect, and photographers manipulate and use this distortion to emphasize the subject or an aspect of the subject in interesting ways.

Per your observations, images made with a fish-eye lens can be post processed to make them more rectilinear, but resolution will always be sacrificed in the process.  The wider and more circular the initial image, the poorer the post-processed rectilinear image will be.  Its generally better to start with a good optically rectilinear lens in the first place.

I'm a little surprised that you only value the 8-15mm fish-eye as a post-processed rectilinear lens.

No.

I defish less than 1% of my fisheye shots.

And fisheye isn't distortion any more than rectilinear is distortion. Both are projections of a spherical world onto a flat plane and so both introduce a distorted view of that spherical world.

The 8-15 will largely be used as a fisheye because I don't like rectilinear distortion when wide angles of view are used. There are times I'll use it as a wide rectilinear at its long end for 16-18mm equivalent rectilinear images, but I suspect this will be under 5% of its usage.

My fisheye shots are hardly ever used as "effects" shots. If my undefished fisheye shots jump out at you as fisheye images, then I didn't shoot them correctly. Some are obviously fisheye, but that should not be the most obvious thing about the images. Some you can't tell are fisheye at all.
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Lee Jay