In LF the max aperture has more to do with image circle and brightness for focusing (and the size of the lens of course). I doubt the Nikkor f/8 would be any less sharp than the f/4.5.

Regarding center filters, keep in mind they need to be matched to the lens, so it is not as easy as finding a generic center filter that will fit into the front threads. I could be wrong about this but I don't know of any center filters for the Nikkors.