(I've only skimmed through the spec, so I may have missed something)

Am I correct that the only form of depth sorting supported is z-buffering?

Why no bucket sorting mode? This is *far* more efficient for software implementations, and even for simple hardware accelerators (worked well for the PSX!)

Ok, bucket sorting ain't perfect - you need a fair bit of application-specific z-biasing to get clean rendering. But it's way faster than a software z-buffer....