Even then I would go for powered ones unless you have good quality riser cables. Perfectly possible to burn the riser cable itself with cards that are pulling upwards of 250w unless they are of good quality. If I can find one of my shit ones ill post a pic of the difference in thickness of the individual cables of a good vs bad riser cable.
GPU will never (or is not allowed to) pull more than 75W from the PCIe bus. That is the limitation of PCIe standard, all the extra power is being pulled from those PCIe 6/8pin connectors.
Riser thickness or quality doesn't really matter in this. The thicker one is probably just having more insulation around the actual wire inside the cable.
7 Series cards seem to have some software issue in Windows if you go +5 GPUs. You can do 5 with 12.6 catalyst, but after that not sure if it's even doable. I went for 7970s since I can pack 5 of them in one rig and that runs nicely with a 1500W PSU while having plenty of room for OC. 6x7950 would have been interesting option too, but maybe not for a Windows machine.
