in my opinion, I'd attempt to crank up the static pressure in the case, remove the pci slot covers between the two cards, and focus on front to back airflow in the case.
I'd ditch the hdd cage, and mount your hdd in the 5.25 inch bays with a 5.25 to 3.5" adapter, to get better flow from the front of the case.
try adapters like this
http://www.newegg.com/Product/Product.aspx?Item=N82E16811993004The people that mentioned the inability/inefficiency of your cards to dump the heat outside of the case, are most likely referring to the fan design using axial fans rather than a centrifugal/centripital blower.
In my experience, axial fans are simply spreaders to draw the heat off the card, and don't do well with alot of static resistance against them.
In order for them to really move any significant heat, or attain any reasonable amount of airflow, they need to be assisted with positive pressure backing them.
Axial fans don't typically deal well with alot of resistance, such as blowing through long runs of tubing/piping.
This is where the static pressure comment I made earlier comes in, use fans such as the corsair sp120's that work well against a harder resistance to pressurize your case, and cause heat seepage out the back.
If you had a blower style fan setup on your card, like the ones on my 7870's, you'd probably fare much better in that situation, sadly those don't seem to be used on any of the r9 series cards short of 290x's.
this is the type of fan I'm talking about:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814161404with blower style fans rather than axial, my 2x 7870's per case never cross over 62 celsius @ 50% fan speed. why do they use axial fans...sigh.
tl;dr, i'd suggest swapping case fans for those that work well against high levels of resistance (noctua f12 or corsair sp120).
try to ditch the hdd cage and move your drive up to an empty 5.25" bay, so you have less objects resisting front to rear caseflow, and consider popping some 4.5" holes in the side panel to add a couple more gpu directed intake fans.
also your cable management looks pretty restrictive, try to minimize objects that your intake air has to blow past, as obstructions like your kraken of cables will only decrease air velocities.
positive pressure airflow with the pci vents being the only escape will crank the heat out the back, although if I had my way I'd lay the case on its face to let the heat rise on its own out the pci vents.