"Another thing you have to take into consideration is that while the lens itself may provide X amount of coverage shutter mounting may reduce this coverage."
Shutter mounting lenses does not change the lens image circle if done to the proper lens spacing pecs. If the lens spacing is altered this can and will effect the size of the image circle and cause other problems with abberations.
If you have a lens shutter mounted and the image circle is altered then most likely the lens spacing has not been properly matched to the original spec. I think commonly extreme wide angle lenses are spaced very tightly and when you shutter mount them this tight spacing might not be possible in a shutter mount, the lens spacing is altered thus effecting the lens image circle. Shutter mounting a lens alone should not have any effect on the image circle if done properly.