x265 is considerably more efficient than x264. I just re-encoded the entirety of my copy of Farscape, from the same Bluray source discs. Here's a comparison of the final file size for each copy. Settings were as follows:

Both copies were encoded with an RF of 20, 1080p, framerate same as source. The #x264 copy only had a 160 kbps AAC stereo audio track. The #x265 copy included that, but also included an un-touched DTS-HD-MA surround sound track, as well as commentary audio tracks.

#Media