I find this interesting, I had just assumed that 280x was best due to everyone saying it was next best to 7950s. Obviously this doesn't consider power consumption, but i believe the 270x are pretty light on power anyway. But obviously you would need a lot more cards to make up the same hashing so maybe power consumption is a big factor.
You fill your mobo on all 6 slots with 270x. Then what? You need to purchase another rig for the other boards. Some people have bigger budgets.
Also, your 7x290x setup needs more PSUs...