Interesting, your numbers look to be quite a bit better than the 30% improvement I've heard is the standard rule of thumb for how much improvement to expect with h.265, and on average for me it's been closer to 20%. I think h.265 starts to show much more meaningful improvements as you start using lower bitrates, and the specific type of media probably matters a bit as well.

I do most of my encoding on an i9-13900k. I've also noticed that h.265 seems to be quite a bit more likely to trigger the smt related crashes in ffmpeg, which makes it a pain when I've queued up a lot of things to encode over night.