I don't know about 5.3K. I haven't tried it yet. I may not, with my antiquated system. It is straining to work with 4K. My take is that while we are looking for the ultimate high-resolution images, we lose sight about the fact that the human eye can only discern so much. Also, how will our work be used? And, at what distance?
I imagine if I wanted to spend about $20.99 per month to get Premiere Pro CC, and bought the newest, latest, and greatest PC system out there, I could work with H265 videos without any problem and bypass Handbrake altogether. I am running the CS6 version of Premiere I bought (the last boxed version, circa 2013), and have no additional costs. If I was making a killing with my videos, that would be a no-brainer. But from a business and financial standpoint, it makes no sense to spend any more money than I have to, since it doesn't bring in that kind of money.
In short, my version of Premiere Pro is too old to run H265, even if my computer was more compatible. If you are running Premiere Pro CC, I would assume you could run H265 5.3K videos, if your computer was more capable.
Even with all the extra work I have to do to output edited videos, I am happy with my GoPro!