If one desktop segfaults due to "hardware related" problems, and the other desktops have incorporated work-around patches, then it's still a bug in the desktop. Pointing fingers at the hardware doesn't help. The segfaulting desktop in question doesn't work. Period. No matter whose fault it is.
How about finally fixing the default plasma-desktop so that it doesn't segfault all the time? It seemed to be quite good in 4.7.[0-2] but with 4.7.3 it is already as bad as before.
I use the open source radeon driver on a HD 6550 and I really welcome the performance fixes. At the moment I don't use the compositing at all because I don't want kwin to idle at 10-15% cpu and I don't want it to need > 25% cpu just to display a non-moving firefox. OpenGL 2 Shaders are not feasible to activate: While moving windows they really, really lag behind the mouse pointer.
On the "mid" power profile the desktop effects are quite horrible on OpenGL and OpenGL ES 2.0 and usable on XRender. When switching to the raster graphics system OpenGL finally looks quite well, but the cpu usage is really not nice, even when idling.
It's a bit sad to see such an advanced window manager still fighting with this sort of performance problems but hopefully 4.8 will fix this.
Gentoo's developers patched kwin 4.7.3 by backporting fixes from kwin 4.8 to fix these issues. It is quite solid.