In wayland buffer sizes don't always match the output resolution size.
This can happen due to wayland high DPI support, or potentially wl_viewport (when that gets added) where a video player/gamer will render content in whatever size it's most comfortable with and we adjust for the screen.
Right now we just use a regular copying shader with linear filtering.
Kwin does have a lanczos filter that's used during some effects, but not during regular scene painting.
That might be worth enabling. Alternatively other scaling options might be better for the 2x case We might want different min and mag filters.