Also, using a 3d denoiser on B5 will drop the bitrate a huge amount without trading off much quality. There is significant grain in those (though seasons 1-2 were worse than 3-5 IIRC).
If the videos are to be played back on a player that supports video filters, you can add some noise back in on decode, which is particularly useful for non-HD sources as your eyes will often interpret noise as detail. Back in the h.263 days I used to do this as 1) h.263 was much worse than 264 or 265 and handling noise and 2) disk space was more expensive, so I couldn't just up the bitrate. I still have some vlc settings that can do that for playing old DVD rips. Today, unless you plan on streaming the video, I would recommend just upping the bitrate and using conservative enough denoise settings to not need it.
At 1080p, you can lower the quality to around 22-24, and get near blue-ray output... the flatter the input (animation is a great example) the bigger the difference. I'm seeing encodes at 40-60% of comparable blue-ray.
The down side, is it is significantly slower, and the quality of the x265 version of the codec is much better than say the nvidia x265 encoder, which is a shame as it's much slower too, so it becomes kind of a non-option for archives imho.
If you are encoding new media for your library, definitely go x265, it's worth the file size saving (so long as your playback devices support it well enough), but I wouldn't go back and re-encode it all, it's just too slow for now... maybe if x264 gets some CUDA support, that would be really nice (recently upgraded to a 1080 GTX).