Just thought I'd come back to say after a fresh install on Mojave, things are working really well.
My export times for h264 have gone from 45 minutes for a 1hr clip to 12 minutes with Apple Hardware h264 enabled. Sustaining such an operation and the card is only at 55 degrees, it does seem 60 is the magic number for fan spin. I think my new fan spin is potentially my PSU, it's only 500w.
There's only two problems I am experiencing:
1. Pink glitches on boot screen, have read this is just how it is
2. Trying to watch an 8k 60fps video from youtube creates stutters at very regular intervals about one every three seconds. Not sure if it's VP9 or something like that, but it was doing this on High Sierra and on this fresh boot. I mean, I have a 1080p screen so I don't mind it was more of just a benchmark. It could also be bottleneck of my system, i7 6700k, 16GB RAM, all CPUs hit 100% and the card doesn't seem to be doing much on the meters.
but apart from that everything is great. All three screens (1xDP, 2xHDMI) are working a treat, and Final Cut Pro is a lot faster.
Highly recommend jumping ship from nVidia if you're waiting, and if like me you're worried about the noise, don't be. Using Pro Tools all day, no extra fan noise from the card. I can now hear it come to a complete stop as soon as Mac OS Login Screen shows up.