For the +III, flash to the latest bios. After doing that the +IV correctly shows all stats (C2-566, 8.5*66), should do the same for the +III.
No matter what multiplier is shown in the bios, the system will use the 8.5 multiplier. Just set the fsb to the speed that you want and save/exit. So...if you wanna run at 850mhz, just set the fsb to 100. It doesnt matter what multiplier is showing. (Although I would leave it at 8, so that the spec.s look closer to reality.)
And yes, Soyo is painfully slow in releasing bios updates.