I'm entry level where digital video production is concerned, but a guru of music production and mac configs/mods. With that said:
Yes, Cuda is supported on the rMBP, in fact Nvidia has their own preference pane for this. I've used Cuda while demoing Octane Render, which definitely prooves that the GPU can run laps on the CPU in rendering.
The CPU between the cMBP and rMBP are generally the same, though better thermal design of the retina helps it perform better.
I maxed out everything on my rMBP, most importantly the ram and cpu. You can change the SSD later if you want to save some bucks. I DO NOT regret spending more for the retina. Its lighter/thinner, has an extra thunderbolt port, and gets rid of the optical drive which serves better as external since rarely used. The main advantage of the iMac is double ram capacity, if that matters to you.
I use the Pegasus J4 with Crucial SSDs for audio-read purposes, really like it. I could add another when I get serious with video, but for now will suffice with a portable USB3 enclosure from Orico.
A good question is, can you hold out til september??