I had a 24mm fisheye and it was not very sharp even on 28MP (found it as new stock as late as ca. 2007). Same was for my copy of 35mm AF, though there is also a newer AF D? version of it which should be better.
Anders, you're the first person I've come across who didn't find the 24mm fisheye outstanding. My sample certainly is, even wide open. You must have had bad luck!
Re. the 35mm: the C, N and AF versions are all the same optical formula, bar tweaks to the multicoatings. The AF D version is probably different - I haven't seen that design in detail.
The C/N/AF version has pronounced field curvature. Many people notice this in part of their images and say "I got a bad sample". But if they conduct a careful test and the effect is radially symmetric, then it's just the design, not a particular bad copy. BTW stopping down reduces the impact of field curvature.