I just upgraded my GTX 660 Ti (2gb) to an 8gb GTX 1080 as I noticed when editing 4k the GTX 660 TI was using all the video ram and figured the 1080 would definitely make a difference. The upgrade to the GTX 1080 has shown Zero enhancement to rendering/exporting speed and little enhancement to realtime playback in the timeline (I tested some Boris FX BCC filters which are horrible at real time playback and take forever to render and export, more below). I have ran several tests rendering the timeline and exporting, the results are very similar to what I got with the 660 TI.
A 9 minutes timeline of 1080p plus 4k downscaled to 1080p is addition to lumetri color grading took about 10 minutes to export on both the GTX 660ti and the 1080. My processor is limited to PCIE 16x2.0 but that is still way more than what the 1080 can output. I did a screen capture while rendering the timeline which show maxed CPU while GPU hovering between 0% to 10% load, I tried to capture it closer to 10% as it was mostly around 3% GPU load. The CPU is doing most of the work. Ram never went over 17gb. Hence always render the timeline before exporting.
Raid 0 for media, Samsung EVO 960 M.2 for export, Samsung EVO 850 for OS, SSD for cache, HDD for Video Previews and a separate HDD for Audio.
As a sideline note, their was no difference in export times between a regular HDD and the Samsung EVO 960 as export drives.
Concerning Boris FX BCC and the new GTX 1080, I did two additional separate tests with the same timeline but added an adjustment layer with a Beauty Studio for one test and noise reduction for another test without Beauty Studio, the results where insane, first it took an hour to re-render the 9 minutes timeline so I can playback without dropping any frames, second it took TWO hours to export the 9 minutes timeline with either effect. To Boris FX you have some great plugins, however, please instead of adding new filters, work on enhancing playback, render time and GPU utilization. Even though I bought BCC and upgraded for the last few years, I really haven't used it as much as I would like to because of the time it take to renders and export, it is very difficult to get real time playback unless you drop to 1/4 resolution (even then some filters still drop frames heavily) and at that resolution it get's difficult trying to make decisions when applying filters like Beauty Studio or noise reduction on what will look best in the filter settings until you render the timeline for real time playback and if you decide to make a change in the setting you will have to render again. For a small business like mine that tailors to small budget project the excessive time becomes a burden an the budget and that is my main reason for not utilizing as much as I would like to. I do love their transitions though and use them a lot, after all rendering a two second transition is not going to delay the production for hours like the effects do. In summary Boris FX BCC has poor utilization of the GPU and rendering/exporting the timeline never utilized more than 10%. of the GPU.
In conclusion if you have a Quad Core CPU there is little return if any when it comes to video editing from upgrading to anything above 4gb GPU (even for 4k, if you are shopping for a GPU for a new build I would recommend the 4gb 1050 Ti or the 1060 6gb anything beyond that for a quad core based on experience is a waste of money for Premiere Pro. . The CPU is the single most important component of the System and it is the first to bottle neck before anything else assuming you are using fast drives. There is no need to upgrade your Ram beyond 16gb as the return is minimal unless you have an 8 core cpu then I would recommend 32gb. Would like to here other feedbacks based on your own experiences with your specific system.