If you're going to be spending 3k or more on the ancient 6,1, and want to buy new, just get the latest and greatest 5k iMac maxed out. Single core smokes what the old Xeon in the 6,1 can do...also multi core...you're in the same ball park (especially if it's the 4 core 6,1). The Dual D700 computes pretty close to the R9M395X. Plus you get a pretty rad screen. It can certainly chew "some 4k", as you say, with ease. It was built for photographers, designers and light/moderate video editing.
Benchmarks don't provide much real world analysis, but I have posted my compute (gpu) and cpu results for both a fully maxed 8-c 6,1 and a fully maxed L-15 5k iMac.
Late 2013 Mac Pro 8-c (Dual) D700 - 64GB Ram:
Single Core: 3671
Multi Core: 21348
Compute GPU: 92057 (I believe the test only looks at one of the cards, but in the real world, I haven't met many apps that can really pull usage of the dual card very well. Most of the time, one is just sitting there chilling)
Late 2015 5k iMac 4-C Mobile R9 395X - 32GB Ram:
Single Core: 5076
Multi Core: 15968
Compute GPU: 86177
For what it's worth, as a Premiere Pro user, I have found little to no differences in the overall performance of the two machines. In fact, the iMac's SSD is significantly faster...the only down side to the iMac is that mobile GPU. BTW, Pr doesn't touch that dual GPU setup often, really just during encoding...pretty much a waste of $ if you ask me (for a dual GPU setup, i'd rather have dual CPU)
Let's be honest though, new desktops are bound to drop soon. Apple has historically never let this much time slip between iMac updates.