After the gradient work I profiled Nimbus again and noticed there were still some fallbacks in action.
I had forgotten to register MaskFill primitives for argb-pre surfaces, and after adding those ... I faced the usual problems:
Note that they are not equally scaled, so in reality the size should be quite the same.
The two visual problems are the white area below the knob, and the different outline of the up/down buttons.
Also the gradient looks a bit different, I am not sure wether this is a problem in the pipeline or just XRender's different gradient implementation.
Overall performance is quite good now and with some planned enhancements for MaskFill (fast upload paths) it should be become even better, well at least something encouraging.
Nimbus seems to be an excellent "tool" for testing the more advanced features of the pipeline ... it seems there is no bug it does not catch ;)