I would not use one of these personally! i would imagine because of the speed of pcie x16 the data that is sent of the extender could become skewed because it could be arriving at different times to data in other lanes.. I would imagine there would be a definate decrease in performance.
is there not another slot lower down on the motherboard?
I would personally cut a hole in the case and turn the psu upside down (to reduce heat making sure there is feat raising the case off the floor) then move the cards further apart on another slot if available (you can buy longer crossfire cables) but apart from that you could always sell your motherboard on ebay and buy a different model.. but that might be a bit far fetched!
p.s buy some cable ties as i think you could improve your temps by atleast 2 degrees!
Turning the PSU upside down and cutting a hole wouldn't do anything really. It exhausts out of the case, it isn't blowing hot air on the card.
And cables aren't the issue here. Like I said airflow isn't an issue, I have 2x120mm fans on medium (high shows no decrease in temps, while low shows maybe 1C increase over medium) blowing directly at the two cards. The cables aren't in front of the card impeding the airflow, but rather bellow them where nothing is going on. Even taking off the side of the case does nothing to lower the temps. The problem is GPU1 is so close to GPU2 it has a hard time drawing air in through the blower.