What I posted was based on what the OP said in a prior post, which led me to believe that the camera that he was intending to use the lens in is one which will not allow movements:
Originally Posted by ChuckP
What he wrote, in part, was, "This is a DIY 6x17, so at the point to pick a lens before finishing off the nose cone..."
The point I attempted to make was that I thought the OP ought to consider a longer focal length than 90mm for his fixed lens, non-movement camera. When I tried that format, I found that the 120mm focal length was much more useful than the 90mm. We can agree, I hope, that is a subjective conclusion and there is no basis, other than individual preference, for his choice of focal length for his camera. I simply suggested he ought to consider another.
If I am correct in that the OP's camera will not have movements, there is no need for him to use a lens that has an image circle larger than 179mm, the approximate diagonal of the 6x17 format. A 120mm f5.6 APO-Symmar has an image circle large enough to cover that and using a larger, heavier, perhaps slower and more expensive lens that needs a centerfilter, such as a 120mm f8 Super-Angulon, is not necessary.
If I am wrong about the coverage of an APO-Symmar or about the intended use of the lens in a camera that does not have movements, then I stand to be corrected. On the other hand, if I am correct about the coverage of the APO-Symmar and that no capability for movements will be available in the OP's camera, then there is no reason for any concern about the fall off in exposure for the APO-Symmar as opposed to the fall off in a Super-Angulon. For its smaller angle of view, the fall off in the APO-Symmar is negligible and, as you recognize in your post, above, it does not need a centerfilter.