For video editing, for that matter really anything, you should see no substantial difference from a properly upgraded 4,1 or 5,1 compared to the 6,1. Trust me, real world experience talking here. I run my business at home on my trusty 12 core 3.33GHz (32gb ram, 4gb GTX 770, PCISSD) 5,1. At work, they got me on a crummy 8 core 3.0GHz 6,1 (64gb of ram and dual D700). Honestly, I see
no difference at all. I'm running apps like Premiere Pro, FCPX, Photoshop and Illustrator all day and night. The difference is...my desk at home is nice and neat while my desk at work has thunderbolt drives literally all over the place.
Save your hard earned money and get a PCIE SSD or something similar in your old mac pro as soon as possible. The biggest bottleneck on the planet is spinning platters. An upgraded video card would be a must as well. MacVidCards has an excellent selection in his store if you want boot screens and full PCI 2 speed in Windows. I highly recommend his services. The old champ also hooked me up with an awesome Bluetooth 4/WifiAC card. My 2010 beast is truly as modern in 2015 as I think I'll ever need it to be. Proof is in the pudding:
https://browser.primatelabs.com/user/ryanthewebber
Also, from experience, FCPX works marginally better with an AMD card. I've got a flashed AMD 3gb R9 280X somewhere in my closet collecting dust...PM me.
In regards to the Retina iMac...the advantage they have over any MP is the single core speed. A lot of apps out there only naw at one core. So, theoretically, the faster your core speed is, the faster your computer. But I mean really...in my opinion I don't think you would see enough substantial real world difference to justify the cost to get into a Retina iMac...especially when you start adding in the Apple price to add an SSD and the better video card.
