The main problem of this MR is that it breaks Vulkan spec. The main rule it breaks is that according to spec, we should explicitly specify that we plan to use external memory when creating VkBuffer or VkImage. The problem is that we don't know if we will until we bind memory to the buffer. I tried some workarounds for that (delaying actual buffer creation, recreating buffer when needed). I got it mostly working and passing tests without validation errors, but it was more invasive than I'd like and still had other problems. I'd rather avoid that if possible.
For what it's worth, I think it's a bad idea to intentionally violate the Vulkan specification.
What makes the aforementioned workarounds ugly or difficult?
If this isn't going to expose optimal performance anyway (I at least think two games are not really enough to accurately measure a difference), I would assert that we should do something slow yet completely safe, like copy the whole buffer, as with GL.
Failing that, I'd just say it's not worth merging this. But then again, I suppose I also don't understand the rush to merge WoW64 upstream at any cost of performance or correctness, without first addressing those costs.