I see a maximum of 1 Ampere in my 7 days timeline.
P = U x I
P = 12V x 1A
P = 12 Watt
But what are you talking about? 300 Watt total combined? The Mac Pro PSU has 5 rails, but no one can really say, which rail is for what. If you want a GPU who need over 225 Watt it is better to use a Aux PSU.