After fighting a bit with it, I was able to draw my first glyphs using XRender.
For now a lot is missing, the most visible missing part is correct positioning of the glyphs.
I am still not 100% sure wether XRender is flexible enough to fullfill Java's requirements in any case, but for now there are other things to worry ;)
After all text is one of the critcal areas where I expect most improvements.
Currently lcd-antialiased text is implemented with readbacks (not sure about grayscale) ... well ... even normal non-antialiased text is rendered with some kind of software-rendering, but without readbacks so its pretty fast.
With XRender modern GPUs have the ability to accalerate even lcd-antialiased text :)
A mistake made me upload the line-circle-picture made by the reference software implementation, so all looked well. In fact, XRender-lines look for now quite ugly, like they would not be of consistent width.
Carl Worth pointed out a solution by John D. Hobby, I'll continue with text/glyph-rendering for now but thats definitivly something I would like to have been done.