Well, there will be a speed advantage to using C over Blitz Basic on the Amiga, depending on how it's used. Blitz supports OS calls ans assembler, which I imagine is not going to carry much of a speed penalty, but other Blitz functions will.
Having said that though, I code in C when I have to, and Blitz Basic when I want to, as with Blitz I personally find it easier and quicker to make progress and gain some satisfaction in the process. C is one step closer to how a computer works internally, hence the speed gain and tight control, but also the longer and more frustrating (for me anyway) development time.
Anyway, for your original topic, AmiBlitz as I understand it still contains a great deal of 68k assembly code, and so a port to Aros would be extremely difficult indeed. Rewriting the compiler from scratch would probably be easier, though if Aros had in-line, transparent 68k emulation like OS4 and MOS have it would solve this and many other problems...