The main factor I see in apparent sharpness of paper is related to the grain of the substrate (not the silver halide grain but the actual formation of the paper base, the "pores" for lack of a better term).
I was nattering about this with Lee over coffee the other day, specifically in relation to the Eliot Porter show earlier this year at the Amon Carter (which received the bulk of his estate).
What struck me about Porter's 8x10s (from 4x5 negs) wasn't a matter of extreme sharpness. It was the color - and I don't mean "bowl you over and assault your eyeballs" color. Just the natural, subtle and faithful color.
As for apparent sharpness, I didn't see anything that couldn't have been duplicated with medium format and possibly even 35mm.
Hear me out...
Below a certain size the grain of the paper interferes with fine resolution. For example, we were looking at a print in the 5x7 range from one of Don Miller's 4x5 negs. I was certain that if I'd had a loupe handy I could have read the name on the mailbox. I was also certain that name would appear to dip and rise with the grain of the paper. However, in an 8x10 or even larger print the name on that mailbox would actually *appear* sharper because with enlargement the paper grain becomes less a factor - it doesn't increase in proportion to the print size. Paper grain remains of a relatively fixed size, tho' this varies of course.
So up to a point we should be printing at a certain ideal (yet undetermined by me) size that would overcome the grain pores of the paper, yet would not exceed the limits of the negative.
Make sense? If not, feel free to dig in. This is just my pet theory backed up by nothing more than personal observations.