I just finished up one of these for a client today. Running macOS 10.13.6 w/ firmware v138.0.0.0.0 from the Mojave DP8, the original 8 x 2.26GHz config scored 2102 on single-core, and 11983 on multi-core (GeekBench 4). After upgrading to Dual X5690s, single core went to 2959 and multi to 19,542 or 24,582, depending on whether I had the RAM running 48GB triple-channel @ 1333MHz or 52GB double-channel @ 1066MHz (he had 2 extra 2GB PC3-8500 sticks).
That difference seemed excessive to me, and I doubt it'll manifest itself that significantly in real-world usage, but I'm going to ask him to possibly try some Premiere & After Effects rendering comparisons between the 48GB & 52GB configs.
We also upgraded him from a Sapphire Radeon HD 7950 Mac Edition (that card is gorgeous, BTW) to a Sapphire Pulse RX 580. OpenGL scores went from 83109 to 136021. While this GPU performance boost may be somewhat incremental, I think the HEVC support that it looks like the RX 580 will get in Mojave is going be a huge boon.
I think a Hackintosh would be fun and interesting, but I just love the Mac Pro towers...they're beasts! And thoughtfully and functionally designed beasts at that. We may be (hopefully) coming into a late renaissance for these machines w/ the support and attention that Apple appears to be giving them lately w/ not only support for, but actually *recommending* the RX 560 & RX 560 with Mojave, plus all of the firmware updates we've gotten w/ High Sierra and Mojave DP/Beta.
You don't say what specific type of video you'll be editing or what tool to edit it with, but I think the 3.46GHz clock speed and 12 cores to play with definitely give you a fighting chance, even on some H.264 4K stuff. And hopefully the AMD UVD support will help that (playback) in Mojave as well. If not, you can always transcode the really hard stuff to ProRes— quite easily in Final Cut Pro X.
Just re-read and saw your last line. Go get you an inexpensive, high-value cMP beast and enjoy building it into the most it can be!
HTH,
Fred