funny you should mention this....
i was using a G5dual 1.8 yesterday (WOOOO) which actually had this option in the energy saver tab of system preferences!
you could chose between low/auto/full performance.
never seen it before!
yeah they removed it on snow leopard
but even with those settings, you can't get to the main one you need to change
its in firmware somewhere and changing it is dangerous / not possible