The only dungeons that 10 would help over 8 is LBRS/UBRS, as the others are capped at 5 players. As far as farming, MMMMP is pretty damn good at farming anything, the only thing more people would help is E'ko farming. In the amount of time you'd spend leveling another 5 set (if it's just to help gear your main 5), you'd have more than enough time to grind out prebis.