if noise is not a problem (think You get quite a lot of it from Your GPUs) get pair of front fans + bottom one (You can leave top & back without fans) something like
Noctua Industrial would be good idea to look for. These are hands down an overkill with all their protection, but the reason behind recomending them is that You can calm those down if You wish. At max (2000 or 3000RPM) they will generate a lot of noise but also airflow.. - this move with 3x fans to suck air in should drop Your temps few degrees at least (or raise render speed a bit).
Also You can take a look into getting Milled vented panel for Your corsair case (it's lit less restrictive, providing better aiflow) - Not so good investment as those fans, but it might help too if You have weaker fans =)