We are presently replacing our iMACs for the top model in the range, which I think is the one you are looking at. The only question I can answer at the moment is that to get GPU acceleration to work you do need the
NVIDIA GeForce GTX 675MX graphics processor with 1GB of GDDR5 memory or Configurable to NVIDIA GeForce GTX 680MX with 2GB of GDDR5 memory
and you may need to hack the cuda.txt file to make it work. (Sorry I don't know where it is on a Mac.) With this graphics chip, some processing is offloaded to it rather than the CPU.
I'd definitely opt for the i7 as any video format above HDV in quality, may well need it.
I have a 2011 iMac with 1gb of VRAM on an unsupported GPU (AMD 6970M) so I can't really tell you how the new one runs although I will be testing it on a friends new iMac in the next week or two. With Premiere Pro CC, you don't need any sort of hack to enable GPU acceleration. It does help speed up moving around your timeline, etc. However, I have some delay issues which I believe are related to my unsupported GPU. The newer iMacs have supported NVIDIA GPUs and my feeling is always, might as well get the best one if you plan to use your computer for editing a lot. A good deal is to get it from Apple refurbished. I was always afraid of refurbished stuff, until I realized Apple's refurbished products are often perfectly working products that are simply returned after a customer has them for a few days. They are thoroughly tested, AND, most importantly, retain the same warranty as a new computer and are Apple Care-eligible. So why not save several hundred? Bumping the RAM up always helps speed up editing and Premiere Pro CC seems to suck the life out of RAM. I go from 16gb to 50MB of RAM within an hour sometimes, depending on what I'm doing. So more is better there too. Here's the link to a good one:
I'd bump up your RAM at macsales.com instead of apple which will save you lots as well.
Finally, I think the iMac is going to get a small refresh of sorts in the coming weeks with the latest Haswell processors and maybe a newer GPU. Not a huge update probably but if you wanna buy full price, it might be wise to wait til later this month (that's when rumors point to the refresh).
I know I didn't answer the primary question you were asking but hopefully this info helps should you decide to upgrade anyway.