SSD is not what you think
1.) with wayland Kwin is THE COMPOSITOR and THE SERVER(that Talks Wayland Protocol) and the same apply for mutter/e17/etc.
2.) SSD vs CSD is basically this:
SSD: Kwin and company allocate a Fixed set of textures in a buffer that will be composited with the application texture FBO
CSD: Kwin will allow Qt/the application to pass a texture/s that will be composited in the main application FBO
3.) Kwin and company know always where the max/min/close/titlebar/etc are no matter if CSD or SSD are used, the only question is who texturize those items before the frame get rendered Kwin or the aplication
4.) Wayland dont handle decorations(for the billion time), in the worst case wayland could only pre define a SHM/FB buffer where you can put the decoration textures in a fixed fashion that will be forced to any application FBO if not empty, just to save some calls between CPU/GPU