Also keep in mind that plex only does hw encode on nvidia, not decode (silly omission of a flag in their ffmpeg build). I let them know about it a few times a long time ago but they dismissed it by saying they don't officially support nvidia.
Plus, most consumer nvidia cards are limited to two simultaneous transcodes.
Although I tested all sorts of hw transcode options, I'm still using sw transcode on my production server, which can handle at least 7 simultaneously.
I'll probably consider switching to hw transcode only when hevc content becomes ubiquitous, because sw transcode won't cut it for that.