That's correct, xsubpp wasn't improved at all. I did do some cleanup of code (adding "use strict" to some of it and lexicalizing variables) but that's about it. The main reason I wanted to stick it in a module was so that I could call its code - which after all, just reads in a file and writes another - without calling a subprocess.
I also agree that there are certainly gems (or necessary kludges, in some cases) from MM's C-compilation code that aren't in CBuilder. I'm depending on people submitting bug reports (and patches for platforms I can't test on) so I can fix them.
Since CBuilder and ParseXS went into the core, I've gotten lots of good patches and suggestions, which will make their way into the next version.