I understand perfectly. I too am always concerned about heat and power consumption from GPUs due to all my previous video card failures. This is why I still hope that Nvidia will eventually release some good drivers. I am not a "fanboy" of either AMD or Nvidia. I simply want what works best and what works reliably.
When I was faced with the choice of either switching to a Vega 56 or 64, the TDP played a very important roll in my decision making. TDP has influenced my decisions in the past, I chose GTX 980 over GTX 980 Ti and GTX 1080 over GTX 1080 Ti entirely because of TDP. I did the same this time around... I went with Vega 56 over 64.
I have always preferred blower style GPU coolers because I didn't like the idea of keeping all that hot air inside the computer case. The last time I had an "open" style cooler on a GPU was with a GTX 680 and it dramatically increased my CPU temps in my old MacPro5,1. Since then, I have always chosen blower style coolers or reference design models.
After I got my Vega 56, I ran it at completely stock settings for a bit and it ran quite warm. Once I was confident that my card was not defective, I took to voiding the warranty and took the cooler off so that I could replace the stock thermal compound with Thermal Grizzly Conductonaut, a liquid metal thermal compound. I detailed my experience
here.
Then, when the VGTab utility was released, I started messing around a bit with undervolting and overclocking. I went through quite a lot of trial and error before I settled on what I felt was a 100% reliable, best performing setting for my particular video card. If you look at my Z370 build thread (link is below in my signature), you will find benchmark results from when my Vega 56 was completely stock. For benchmark results from different tweaked results, please see
this post.
Now, I have an extremely quiet, great performing, and relatively cool running Vega 56. Under normal circumstances (with my window open and/or with TV noise in the background), I can't hear the difference between when my system is sleeping or awake. When the Vega is pushed and it heats up, the fan does ramp up and will be audible but it's much better than it was before I did all the tweaking and the noise is much less unbearable. I can't be more happy with the results.
As a FCPX user, I'm sure you will see great performance improvements with a Vega. Also, for FCPX usage, I don't think it would be as hot as when running the latest FPS game for hours on end. Btw, as a user who has had both Nvidia and AMD, yes, your UI stutters are entirely due to the terrible Nvidia web drivers.