C2758 seems to handle one Plex transcode (H264->H264, maybe with AC3 to AAC) fine. If there's any particular benchmark you'd like me to run, let me know.
Can also confirm no issues with this transcoding on the C2758. I built a SuperMicro 4 drive NAS a few years ago based on this CPU and it has been very solid. Doesn't run well in Docker, but runs well baremetal.