Martin Gräßlin, the maintainer of KWin, has been aware of R300 rendering issues for a while but hasn't had access to any hardware that works with the "R300g" driver. Back during XDC2011 Chicago I passed along an ATI Radeon X1300PRO graphics card to get to him so he could work out these issues.
Martin now has the PCI-E graphics card and has already fixed up two texture bugs and has begun work on GL2 shaders for R300, which previously was completely broken. These are improvements lining up for KWin in KDE SC 4.8.
I met Egbert on Friday and finally received the R300 Thanks a lot again. Thanks to having such a model now in my system I was able to fix one bug specific to limited NPOT texture support and I hope it fixed another one only present on cards without any NPOT support (e.g. R200). We will know that after the release has been shipped.
Also I started to work on adding support for GL2 shaders for R300 which was completly broken. This will hopefully end up in 4.8.