For those that are interested, I've noted the following trend for my two cards:
R9 280x - 8.33 Watts/MH/s - 20.4 MH/s total @ 170 Watts
R9 280 - 8.73 Watts/MH/s - 19.4 MH/s total @ 170 Watts
These are at the wall readings. Thus for the $50 price difference b/w the 280 and the 280x, you're getting a little better efficiency and slightly more hashing power.
If you have been able to figure out W/MH/s for the other R9 cards, please post below.
More info on Eth pricing and profitability