I'm not sure whether performance problems exist that only some configurations suffer from. I can only tell you that NVidia's drivers provide excellent 2D acceleration, without "tweaking" anything on my systems.
At least, is there any gpu-acceleration in the combination of Xrender path with the RenderAccel option at all?
NVidia's drivers accelerate almost all XRender operations and it's enabled by default for a long time already, no need for that option. In fact, IIRC NVidia's XRender acceleration performed best in some benchmarks (ask linuxhippy).