that helps of course , (in fact he cherrypicked some things from my kernel, lile LCD timings), but some drivers need userspace libraries that usually come with full SDK. I suppose he cannot share the full source for some reason (a NDA probably), For gamepad he used a different approach, but it is a HID gamepad. The gamepad modes of XD 4.4 can be used too, the driver is in my github.
Anyhow, about Vulkan, only PPSSPP would benefit for it I guess, and maybe retroarch, but that's it. I suppose the biggest improvement would be more android game support. Overclocked seems to be removed from his kernel too (it seems in new RK kernels 4.4x they use speeds in a different manner).
Buy yeah, someone with knowledge, but specially time and motivation could do a 7.1 ROM or even Oreo maybe. I asked GPD several times if they had access to a more modern SDK, but they never replied, probably because that would stop sales (even more, because I don't think it is selling too well) for xd +.