Just adding two cents. I did a couple movies both ways (AVC (H264) and HEVC (H265)) and noticed around an average of 33% reduction in size using HEVC (depends heavily on how much is going on in every scene of the movie, based off the algorithm it appears to use), and could not (I'm talking visually on several outputs, not any 'lab controlled' environment) distinguish a visible difference in quality. Also, I use Plex. The HEVC worked wonderfully on my FireTV sticks (I've got several over several gens). It worked wonderfully for my Nvidia shield (1st gen, in case it matters, fully updated). It worked great on my 2018 Samsung 4k TV as well (Direct app). It played perfectly on my Samsung Note Ultra (S20 version). So for the most part it was wonderful (Direct play for all formats mentioned). The problem comes down to when you can't play directly. The Transcode for any web player, (on windows 10 anyways) without hardware acceleration turned on in plex would peg my cpu at 100% for transcoding (HEVC Transcoding is SUPER cpu intensive, at least for an only IVY bridge 4th gen i7 Desktop CPU), whereas transcoding for a standard avc (H.264) was minimal (18%). IE I could do several simultaneous streams of AVC transcoding while only 1 (and hurting doing it) for HEVC. So, something to consider. All that said, I decided to still go with HEVC as it is compatible for all my streaming devices in the house, and for the windows 10 computers, I just use the desktop app for plex, instead of the web browser, as the space savings (especially with hard drive prices right now) are more beneficial to me.
Just a side note (More Plex notes, but may be valuable for those in similar test beds), I did turn on hardware acceleration on Plex as well and it reduced the load to around 20% on CPU and around 25-32% on GPU (with some occasional 100% spikes, all monitored using speccy and normal windows procmon). That said however, the hardware acceleration was buggy, and would intermittently freeze (nothing showing extra work in the process monitoring, just glitchy streaming, like it was buffering, and no network is not an issue). Also, with hardware acceleration turned on, playing the HEVC and trying to skip/pause/rewind, led to super long (20 seconds+ most the time) delays before it would start again. None of these issues were the case using just the CPU for transcoding (it hurt the cpu, but the person viewing the stream wouldn't notice), IE, if I jumped to the end of the movie, you'd get very fast update time for it to pick up and play (usually 1-3 second delay, which is better than I get from normal streaming providers, trying the same action). Anyhow, just figured I'd include some of my own testing for my uses.