Back when I was excited about Plan9, I saw Tom Duff at Siggraph and I asked him if there was anyone present who would be interested in talking about the new Plan 9 window system, rio (which provides a compositing algebra-based raster graphics layer). He just said "no", basically, which led me to conclude that graphics on Plan 9 was not flourishing, even though the rio model is interesting.
Performance over the network can't really be much worse than X11 for raster-heavy applications. I suggest you experiment with Plan 9/rio in a VM if you are really curious.
I should add that the model of an application that is split across the network (e.g. Rob Pike's Sam editor) offers ways to improve that. I can't speak to the efficiency losses of locally using pipes rather than some kind of shared memory, DMA, or direct graphics access, but maybe inferno has some multimedia-oriented fixes for that, I don't remember.
