I have an update on my Vega 64 upgrade. I purchased a used reference Sapphire Vega 64 (non nitro) from ebay for ~$410 and did some tests with it.
The fan speed issues were present at first, but the kext seemed to work pretty well as a fix. Sadly the fan speed does not scale with the increased heat, it only kicks in after it hits a certain degree threshold.
High Sierra instantly recognized the card as a Vega 64 which is great, and I haven't had any crashes yet + multi-monitor works.
However, I was surprised that samtimbaud was correct about the 'coil whine' sound. It is present on my card as well. It's not incredibly loud, but it is definitely noticeable. I have dual boot with Windows 10 on my system, so I tested it on there too. The coil whine sound is ONLY present in Windows under full load (gaming, rendering), but otherwise it is silent. The problem is in High Sierra the sound plays just by moving your mouse or scrolling (pretty much constantly) even when doing basic tasks or idling. It's not a huge deal breaker, just something to take note of. I can hear the noise through my case which has sound dampening material and no window.