In my experience, BIOS upgrades sort out a few problems and sometimes add compatibility for newer processors. The upgrade on my Asus KV8 Deluxe added compatibility for Sempron CPU's, whereas the bios u/g to my Soltek board seemed to sort out some problems (that hadn't shown themselves to me anyway). On neither/any occasion have I seen any increase in performance.
Perhaps the people who see the biggest increases in performance are those who've been suffering at the hands of a problem that the bios sorted out!