@joevt, card arrived today. After some finagling, I have it working in my 2012 5,1!
First impressions:
The thunderbolt ports on this card lack a bit of quality, so you really have to make sure the devices are plugged in solid. I first got it working with my GTX 1080 in Windows 10, but knew it would be a fail with High Sierra due to Titan Ridge support.
Next, I powered down and replaced the 1080 with my spare RX 580 to upgrade to Mojave. The RX 580 cooler as many people know is a bit taller than the 2 slots, so I had to loose my USB3 card and unfortunately put the TB3 card in the x4 slot 3. Waited forever for the firmware updates and finally got it installed. Booted Windows then over to MacOS and I successfully have a LG Ultrafine 5k and a OWC Thunderbolt 2 ThunderBay4 running! Hopefully new web drivers drop someday and I can toss my 1080 back in so I don't loose this precious PCIe slot.
I've only had about 5 minutes with this setup to test, promised a 'date' with my daughter. Will do some more experimenting on this setup tonight.
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Update:
Home now...starting to tinker again. For the 5k UltraFine: **edit**full 5k 5120x2880 resolution** I have full display controls (brightness, FaceTime camera, mic in, and speakers) from the thunderbolt controller. Also, additional USB ports on the back of the display are working as intended.
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Next up, would love to tackle figuring out how Windows magically blesses this and force MacOS to do the same.