Yup, I'm going to try to setup this first one to intake air from the front and do a test but the problem is you will push all that heat from the GPU's into the PSU, Board, Memory/CPU -- I'm not sure which is the lesser of the two evils.
The kit and its instructions however have the hot air coming out the front.
Just throwing my .02
BTC into the conversation. Unless the fans on the cards can be reversed, you almost have to exhaust out the front of the case. Otherwise the intake from the front fans and the exhaust (Facing the front) from the GPUs will just create some interesting thermal eddy's and not really move sufficient air in the first place. If you're really concerned with air-flow at the rear, you could also add a few of the pci-slot exhaust fans, like these:
https://www.newegg.com/Product/Product.aspx?Item=N82E16835119066I'm not recommending that one specifically, just using as reference.
The case add-on looks great, if I was getting back into GPU mining, I'd pick this up in a heart beat.