I am also struggling (badly) with PPro (currently the latest version 2015.3) and 4K XAVC-S files from my A7s-II.
I get stuttering during playback on PPro timeline (or even just playing a clip off the timeline), even without effects. But it's not constant. It'll play then just start stuttering. If I stop then start the timeline, it'll smooth out for a bit then start stuttering again. If I add Lumetri then it will slightly stutter all the time.
I have a GTX970 4GB GPU. I know that's a decent video card. My CPU is a little old... it's an Intel I7 3770K. I have 32GB of RAM. PPro and all it's mediacache files reside on two different SSDs. The XAVC files are on a WD Black 6TB drives (but I tried putting all the XAVC stuff on SSD and the stuttering wasn't any better).
I am to the point where I HATE editing. It's so frustrating that I have to keep stopping and restarting playback. Might sound flaky, but it interrupts my creative thinking when I can't watch the whole thing at one time!
I also do not want to transcode my XAVC into another format. If I open task manager I can see my CPU is pegged at 100% when I playback.
I want to enjoy editing again! :-) I was thinking of buying a new motherboard with the latest 6950X CPU and getting 64GB of RAM. I think the video card is still good and I've heard that going from a 970 to a 1080 only yields about a 10% improvement.
Any help or advice would be sincerely appreciated!