ok so I got a new 802.11ac router that connects at between 500-800mbps speed on the KS8000. I don't know what its actual throughput is but it should eliminate raw network speed as the problem. In testing on pc some movies were using slightly more than 100mbps but I don't know if that's because it was streaming the entire file when watching on pc (i.e. every audio track). I'm still having some problems on some files with Plex on the KS8000 so it seems there are several issues identified by the forum and maybe others I'm unaware of.
a) the tv/plex client dealing with either specific audio formats like dca (dts) or plex dealing with all the audio tracks bundled into the file. Instead of doing a full rip off my blu-ray on some movies I did a rip of just the video and a single dolby audio track which fixed the problem with stuttering in a few movies. Some people blamed this on dts (dca). I'll try to make a bunch of rips this week to figure out if it's something that specific
b) plex having issues streaming large files in direct play leading to stuttering that is not entirely the blame of the interface speed of the client. It's not easy to determine where to assign fault when buffering problems occur because SmartTV aren't open platforms
c) bad rips. I tried some rips other people made which had serious problems on playback even when playing back on a usb stick. In several cases I ripped the blu-ray myself with makemkv and it worked fine (haven't disentangled enough from the audio issue yet since re-ripping 15 blu-rays is going to take hours)
d) 60mbps limit on native hevc codec playback on KS8000 and some other Samsung TV. It seems like the movies that work best are 45-50mbps video. I have a couple that are 60-61 that have artifacts and/or stuttering. Where it's a hardware playback problem the only solution is to get a different player or just go back to the blu-ray player. Considering I just spent $300 on a new wireless router and only have 15 UHD blu-rays I'd rather not spend another $200 on an Nvidia Shield especially when it's unclear how good the video quality will be compared to the OPPO blu-ray player I have in another room or the native Plex client