As a feeling - the 8-15mm is the best non-water contact fisheye you can use on a Nikon - it is noticeably sharper than other fisheyes.
But it is not as good as the RS 13mm fisheye or WACP. But of course, the images are totally useable for any application.
I was processing some recent shots of the Kittiwake wreck the other day and I noticed that the image quality wasn’t as nice as usual (most obvious as Chromatic Aberrations) and then I remembered that I had used the 8-15mm that day, rather that the RS13 or WACP I normally use for my wide angle. But of course the images are still very nice. It is a small difference - but when you know how it can be, you do notice it every time!
But the main reason for using the WACP is that it has a totally different angle of coverage than the 8-15mm. The 8-15mm is 180˚ corner to corner @ 15mm. The WACP covers 130˚-57˚ corner to corner with the 28-70mm - making it a super versatile option. But I still always travel with a fisheye too - for shooting the biggest subjects (wrecks, scenery, very big animals, etc). I take the RS13mm when splits are not important and the 8-15mm when I expect to shoot some splits or want to use the 16-35mm too (they use the same dome).