So a some hot air is probably pushed towards the back. But if you look at my screenshot of a couple GPUs doing 59C and 66C, I believe it'll be fine. I think the other GPUs with higher temps need a TIM replacement. From reading the newegg.com feedbacks, the Twin Frozr 3 have bad TIM from the manufacturer. Once I get around to getting it replaced, I'm sure I could lower the temps quite a bit.
where do you get TIM replacements from? which ones should I buy? I have 1 7950 that no matter what I do is always 6 -7 degrees over the other ones, probably that is the case too (sapphire 7950 oc boost) also have another sapphire 7850 with heat problems but it's because one of the fans are acting up 
Change the paste. Install the heatsink then remove and check see what pattern the TIM leaves behind.
Also check the thermal pads on the memory and vrm chips and make sure they line up and are not sqeezed on too tight. eg.. one of my R7870s and mobo both needed some work that way..