Ah okay. I think the work required to get this implemented across the stack will still cost a lot more than $11,000, just saying. Also in my opinion 4K monitors at that size are an unfortunate purchase because the PPI is not high enough to look crisp at that viewing distance. Apple upgraded theirs because you need to get above 200 PPI range in order to have it look comparable to print. For me personally, I couldn't justify spending my spare time working on that when other monitors exist that don't have that problem.
4K at 27" is an unfortunate resolution / density. I know, I also have such monitor :(.
That said, 10-15 fps on X11 is a pathological case, something is wrong there. When I ran X11 and played with xrandr, the slightly larger framebuffer had zero impact (outside games) on AMD Vega64.
Usual price point around $1600-$2200.
Comparable 2× monitors start at around $4500, and the cheapest option only supports macOS and has a $1000 monitor stand.
A "few hundred dollars" won't be enough.
At the moment, I just work all day at ~10-15fps using Budgie with fractional dpi on X11, but it's definitely not a great experience.
I used to be 100% in on KDE, but as you mentioned, Wayland support is limited.
Budgie is atm sadly the only option for multi-monitor hidpi that's not using gnome's broken top bar concept.