One issue I've come across is with higher temps the electricical resistance increases in the GPU PCBs and the rig draws more power at the wall to compensate for this resistance to meet the voltage requirements. When one of my 3x7950 rigs was running 75C+ it was drawing just over 1000w at the wall (mind you it should only draw 200w per card).

With an open air case and riser cables you should be able to bring those temps down considerably. Also get a kill-a-watt! You might be able to borrow one from the library if you dont want to buy one, most cities have energy conservation plans and lend them out.

Not only will you prolong the life of your cards, save energy, but can also clock them higher to get over ~630 khs. I have mine clocked at 1100 core and 1600 mem running at 59C with 60% fan speed.