I have had experience with the 290x, 295x2, 390x, fury x and pro duo. There are a few things to consider... First, the hashrate will decrease for any gpu 74 degrees celsius or above. Second, for a non-commercial setup the easiest way to overclock the cards is with msi afterburner with power set to +50 up to 1137-42 leaving the voltage alone with for a 4 way crossfire x a power supply that does 1300+ continuous watts. Third, use the most current mining software for example "claymore's dual eth and decreed" for more efficient mining and increase in profit. Fourth, it helps to have an eth proxy server if you have 2 or more nodes. For example we have a setup with 1 295x2 ( 2 290x ) and 2 fury x all overclocked and it does 133 non-calculated mh for ether whereas another one of our setups with either 2 295x2's or 4 290x's were getting around 116-118 MH. Our rig with 2 pro duos (2x2 fury x) 147 mh a considerable difference. A tip is to leave your cases open with secondary household fans venting the heat from the already 100% liquid cooled system ( can be done with closed loop cpu, gpu ).

I would have to say I have a difference of opinion of the hashrate of the 290x beating the fury x. First, the fury x is more power efficient, stays cooler, and has 4000 cores at the same speed as the 290x with 2800 cores. The fury x also has higher overclocking ability. The fury x definitely outperforms the 290x but its not due to the HBM. A pro duo has 2 fury x gpus with 8000 total cores where as its predecessor the 295x2 has 5600 cores with 2 290x GPU's so the core difference is a noticeable increase in speed in terms of GPU mining especially with the newest mining software.