If you undervolt and downclock the cards to their most efficient (in terms of MH/s per watt) point, then you can easily run them all on a good 1000W PSU. The better question then is whether you will get enough of a hashrate bump to justify it... I can't answer that.

I would not personally do this because 280x's and 7950's like very different scrypt settings, and you are hobbling your rig if you ever want to go full blast.