My pi 2's running rasplex do not require any transcoding at all from the server. Only the clients connecting over the internet do for my setup. It doesn't care how BIG the files are....but what bitrate the device that is streaming the data to can handle.
Because that's Plex to Plex... as long as the receiving Plex has sufficient bandwidth and CPU to not request transcoding, you're good. But, if you're streaming to things that are more format-limited, and certainly to any mobile device, you're guaranteed to be transcoding at least some of the time.