Im pretty sure the Mac Pro power supply is 980 watt PEAK. Continuous is prolly around 800. Notice that nobody builds HD power supplies for the Mac Pro, whereas you can get all sorts of power supplies for PCs; up to 1200 Watt continuous. Some of them even have LEDs and pretty fans.
I would like to build a HD power supply for my 5,1 not because I need more power (single Radeon 5870). My standard power supply is adequate. BUT, if I had 1000 watts continuous I would be operating at a much lower percentage of the power supplys capacity, hence more efficiency & less heat. ALSO, a good power supply would likely have less ripple & noise, and tighter tolerances on the DC outputs (tho Apple does not report this data, so who knows?), hence less stress on the components.
Other advantages would include the possibility of dedicated leads for video cards, SATA drives, even Molex connectors for pretty fans.
I think the biggest problem with porting over an ATX power supply has got to be the two temperature sensors for the (separate) fan speed control through the mainboard SMC. Without these temperature signals, the SMC would likely go into panic mode & destroy the city.
Any thoughts?