Great video reviews, thanks for posting.
I'm no video editing experts by any means, but I do wonder if the huge rendering times compared to the 15" rMBP with Nvidia is because of the lack of support for Intel Quick Sync on the new Iris 6100 GPU? I use a Windows computer myself with an Intel 3770K to convert videos myself, and using Intel Quick Sync is faster than using CUDA. The same app I use for conversion, I was not able to use Intel Quick Sync on another computer with an i7-4790 because it was not updated for the i7-4790.