I agree with Jack about the 4-5X enlargements from 4990 scans. I have a 7600 and 2 foot wide prints from 4x5 look great most all the time. Sometimes I'll get something a little less than great but I can count on it about 9 out of 10 times. I have no reason to believe that 8x10 scans won't do the same. My 8x10 printing experience stops at 24 inches as thats as wide as my 7600 prints.
As far as the enlargement factor for the 4990 goes, I comparison tested a Mamiya 7 6x7 negative scanned on my 4990 and on a top end drum scanner. At 11x14 print size there really wasn't any significant difference - I couldn't really pick one as better. Larger prints did show a difference but not as much as you'd think. I regularly make 11x14s and slightly larger from 6x7 that look great.