Sorry, most of this is information from 2014. OpenCL is widely supported across all effects. It took time, but it's there now. Today, Fury X is the most stable and fastest card for the latest Premiere Pro released last week. You are right that most workloads are CPU driven and hence don't show off the GPU's potential. But drop in a heavy sequence - we are talking 8K on a light codec with a whole bunch of effects, and Fury X shows its might, outperforming even Quadro M6000. (Haven't tested 1080 yet)
Premiere Pro uses GPU acceleration extensively, everything from scaling, colour management transforms to exports. For exports, it even uses multi GPUs. How much your system is CPU limited is determined by the source format used. That is because decoding is almost always CPU bound.
So, if you are reading 6K R3D RED RAW files, it doesn't matter - you will be CPU bound till you have a 20-core system. If you are reading 4K XAVC or XF-AVC files from cameras like Sony F5 or C300 Mk II, you will be CPU bound till a 6-core system. Some codecs are storage I/O bound too. Such as ARRIRAW or F65RAW. These are uncompressed codecs with GPU accelerated debayer, so the CPU doesn't do much work. But you need a SSD or fast RAID0 subsystem to eliminate the bottleneck as they draw to the tune of 300 MB/s. Finally, we have mezzanine codecs like DNxHR, Cineform and ProRes. You can run 8K easily, little stress on CPU or I/O. Here the sequence become GPU limited as the GPU has to work to scale down the 8K to your sequence and display.
It's completely false that you can't edit 4K on Premiere Pro. We have finished multiple 6K RED Epic / Weapon projects. This is the most popular camera being used. Like I said above, with a light codec, 8K is pretty easy on Premiere Pro with a mid-range system. Check out Devin Super Tramp - he has multiple videos showing off his workflow - a lot of 6K footage finished at 4K. On a Mac with AMD graphics cards, by the way.
Finally, Final Cut Pro X is a massive improvement on the archaic FCP 7. But to say it's much faster than Premiere Pro is a myth propagated by Apple fanboys. For CPU limited decode, it's up to par with Premiere Pro but no faster. However, for scaling operations, such as using 6K footage in a 2K sequence and so on, Premiere Pro's GPU acceleration is much faster. It's not FCP X's fault, it's probably a case of graphics card drivers being much more mature on Windows than macOS. FCP X's only major performance advantage was the background transcoding, but that has now been mitigated by Premiere Pro's new Ingest features. Right now, the only thing FCP X does faster is colour correction and stabilisation, but Adobe's Warp stabiliser still produces better results and the Lumetri colour engine is far more sophisiticated.
Having said all of that, you can get significantly faster exports out of FCP X. But there's a bit of a trick with how it works. You see, unlike Premiere Pro which reads files natively, FCP X has a rendering engine, which renders file as an intermediate before displaying it. So if you are just editing and exporting, FCP X is indeed much faster, because it simply accesses the low overhead render files. (at the cost of lower quality) You can of course pre-render in Premiere Pro as well, but it isn't persistent like FCP X.
At the end of it, everyone's going to get different results based on what you are going to do exactly.
So, it's prudent to optimise your system according to your workload.