One think i have noticed editing 4k is i get much smoother playback droping 4k in the timeline and getting it resizing to 1080p on the fly, I would have thought this would make things worse but it's smoother, I've been wondering for a while now if they restrictions are more about how big each frame is decompressed and moving that about rather than the cpu/gpu and there may be a way to better optimise that but it's working well enough for me that I haven't bothered looking further for now.
It's not like wow that's amazing but it works for me, can play and edit 4k fine without proxy, soon bogs down depending on what effect you use but obviously don't have them turned on for the editing part. I'm running the latest drivers and it looks like vegas making much more use of all my resources, regularly get both cpu's maxed out now and usage on the cards is up. Then try going into the internal advanced settings and set the OpenCL Filter memory size filter to a larger value, I set mine to 3000 the 980 has 4 gig and the 780 has 3gig and the 980 started working better. NOTE: These drivers are currently the only method to get full acceleration for NVIDIA GeForce GTX 750, 750 Ti, 950, 960, 970, 980, 980 Ti, and TITAN X Maxwell graphics cards. Source: My 970 didnt support openGL (yaya, different things, bu w/e), a reïnstall of the drivers fixed it. Reinstall drivers, no need for a clean install usually. I had to do a reset of vegas first to get it to pick both cards up, might be worth doing that first. These drivers have been known to solve OpenCL issues with certain applications, as well as providing better native GPU power management for certain devices. Go to solution Solved by Elapo, January 25, 2016. I run and old 780ti for display etc and have the 980 selected for render on vegas