300 Watt is 75 Watt for each of the 4 Slots. 75 x 4 = 300
BUT: You can't get 300 Watt form only one PCIe slot. If you install a GPU you get 75 Watt from the PCI and the rest from the GPU power connectors.
The cMP has 2 x 75 Watt GPU power connectors. If your GPU need more then 225 Watt (1 x 75 Watt PCIe + 2 x 75 Watt GPU connector) you need a AUX PSU or you get the power from other sources like the HD connector.
You think, 5 Watt more is not a problem i will use an Adapter! But there is a problem. The 770 has a 8 Pin connector who can pull up to 150 Watt (2 x 75) and the the power is split approximately evenly to all connectors. 230 Watt / 4 = 57,5 Watt
So you can get:
PCIe ~ 57,5 Watt
GPU con 1 ~ 57,5 Watt
GPU con 2 (with a 8 to 6 adapter) ~ 115 Watt (not good)
But perhaps when you downclock the 770 to 225 Watt (-5 Watt) this can eliminate the 115 Watt problem. But it is better to ask guys like MacVidCards about this.