Quite frankly, between an x265 version from here and x264 from the official publisher, I can't really tell you which version is the best.
Usually, to see the differences in quality on files of these two codecs, I run the two files, pause and stand at exactly the same frame to see the differences (and switch from one video to another).
Unfortunately, for VR videos, it's more simple because we cannot launch two videos at the same time and even if we had two headsets, switching from one headset to another, we necessarily lose the feeling of the videos ( unless there really is a huge difference of course).
Technically, a x265 video can easily be 2 times less heavy than a x264 video and if you push the encoding options fully, you can even reach 3/3.5 times more.
I'm not sure that SLR fully optimizes the compression options, but I have no doubts about the quality of their video compared to the original videos for a much less weight.
Personally, all the videos I had before in x264 and which are available on SLR, I recovered them in x265 and with a higher resolution (as I have a vr gear, I took files in 1440p or 1600p, now , I recover them in 1920p) for a weight that is either equivalent or less (I saved a loan of 5TB thanks to the x265 videos).
And unfortunately, as SLR does not do all the studios yet, the rest, I re-encode them myself (for example, I recovered all the badoinkvr and naughtyamericavr at minimum in 1920p and I re-encode all that, even if it takes a long time , especially since I don't have a very powerful cpu (I7 6700HQ)). But little by little, I see the saving of space that I am doing for a higher quality than what I had before.