Thanks for all the replies. Unfortunately the seller did not respond to my query, so it looks like the MacPro3,1 will not be mine. But judging from some of the replies, and some experimentation of my own, it may be for the best.
I decided to do some experimentation and built a "Hackintosh" with very similar specs to a Mac Pro 5,1. I then dropped in a RX 580 to do some comparisons with. Since I didn't mention it in my first post, my 'footage' is actually gameplay capture from a PC. It's encoded with h.264 at 1080p/30fps. Each file is 30 - 60 minutes long. Nothing too demanding.
For the comparison I used Davinci Resolve and iMovie. I did not do a rigorously scientific test, but just a general evaluation to see how video exporting compared on my MacBook Air vs. my "Hack Pro".
Hack Pro: Xeon X5670 (6 Westmere cores @ 2.9Ghz), 24GB ECC DDR3-1333, RX 580 8GB, WD 1TB 5400 RPM HD, Mojave 10.14.4
MacBook Air 2017: i7-5650U (2.2 GHz), 8GB DDR3-1600 RAM, Intel 760p 512GB SSD
The MBA has Macs Fan installed to try to minimize thermal throttling.
I found that when exporting with iMovie that the Hack Pro and the MacBook Air encoded a little faster than realtime. So 50 - 55 minutes of footage encoded in about 40 - 45 minutes on both machines.
Davinci Resolve was a little different. When exporting the same lightly edited footage with the same settings (downscaling to 720p h.264, a few titles and crossfades, bandwidth limited to 10Mbps) the Hackintosh took a little under an hour and a half to encode the footage, whereas the MacBook Air took about 40 minutes.
That last bit was eye opening. I don't know if I've misconfigured the Hackintosh, but it gets about 14000 in Geekbench 4 and Resolve 'recognizes' the RX 580, so I don't know what's up. I'm amazed that Resolve even runs on the MBA. I thought it required a CUDA or OpenCL dedicated GPU, but it does indeed run on my MBA and mops the floor with my fake Mac Pro.
EDIT: Looking back at some of the previous posts, it seem that Quicksync may be my MBA's speed advantage. But I didn't know Resolve even used Quicksync...odd.
Both Davinci Resolve and FCPX use Quicksync for their decoding of H.264 files; so depending on the compression rate and the type of files you ingest into Resolve, it is apparent from your experiment that your files benefit from Quicksync and to me, this is really not a surprise. Since you have the Broadwell CPU, look below at the benefits you get with Resolve. The Coffee Lake CPUs with the T2 security chip are even more impressive.
"Version 1 (Sandy Bridge) Quick Sync was initially built into some
Sandy Bridge CPUs, but not into Sandy Bridge Pentiums or Celerons.
[6]
Version 2 (Ivy Bridge) The
Ivy Bridge microarchitecture included a "next-generation" implementation of Quick Sync.
[7]
Version 3 (Haswell) The
Haswell microarchitecture implementation is focused on quality, with speed about the same as before (for any given clip length vs. encoding length).[
citation needed] It has seven hard-coded quality/performance levels (called "target usages"), compared to the three in previous generations. The highest-quality TU1 setting is intended to be higher quality than Ivy Bridge's version, and the highest speed TU7 setting should be faster, higher-quality, and more battery-friendly for mobile devices. This generation of Quick Sync supports the
H.264/MPEG-4 AVC,
VC-1 and
H.262/MPEG-2 Part 2 video standards.
[1]
Version 4 (Broadwell) The
Broadwell microarchitecture adds
VP8 hardware decoding
[8]support. Also, it has two independent
bit stream decoder (BSD) rings to process video commands on GT3 GPUs; this allows one BSD ring to process decoding and the other BSD ring to process encoding at the same time.
[9]
Version 5 (Skylake) The
Skylake microarchitecture adds a full fixed-function
H.265/HEVCmain/8-bit encoding and decoding acceleration, hybrid and partial HEVC main10/10-bit decoding acceleration,
JPEG encoding acceleration for resolutions up to 16,000×16,000 pixels, and partial
VP9 encoding and decoding acceleration.
[10]
Version 6 (Kaby Lake, Coffee Lake) The
Kaby Lake &
Coffee Lake microarchitecture adds full fixed-function H.265/HEVC Main10/10-bit encoding and decoding acceleration & full fixed-function VP9 8-bit & 10-bit decoding acceleration & 8-bit encoding acceleration."
However, unlike FCPX, Davinci Resolve Studio (the $299 version) allows you to use multiple GPUs! The best setup would be a iGPU from the Coffee Lake CPU (Mac Mini 2018) with Quicksync with T2 security chip and multiple GPUs with CUDA cores so both work simultaneously to provide even MORE rendering speed for Resolve Studio. In fact, in your case, you should be using a eGPU rather than building the Hackintosh, because with an eGPU can help rendering the transitions and color correction work while Quicksync does the decoding and encoding which the Xeons can not do. This is the main reason why a number of Mac users who bought Mac Pros had to sell to buy something like a Mac Mini or Macbook Pro and get an eGPU until they upgrade their consumer grade video equipment to a much higher professional gear where the files produced will work ideally better with the Mac Pro and CUDA cores.
I use my Macbook Air with the Haswell CPU for downscaling 4k footage to 1080p and even a 2014 model beat the Mac Pro 5,1 using the software that harnesses the capabilities of Quicksync as there is nothing like it in the Xeon nor would the CUDA cores be any help with consumer grade video files which you had working with.
