Plan9 uses a different dialect of C written by Ritchie (his last compiler) than most of today's software. Also, the standard library and programming model are quite substantially different than the ANSI C model. There is something on Plan9 called APE (ANSI Posix Environment), which is kind of like WINE (an emulation layer, not an emulator or compatible ABI), for porting ANSI C apps, but it's built with a rather old version of GCC and is far from complete.
Meaning, most software won't run on Plan9 without major modifications, so bringing a modern GCC/CLANG would give access to better compilation, but really there needs to be more work on APE to get more software over (lots of low hanging fruit there from what I've seen).
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