For what it's worth, the go team's plan is to automatically translate the compiler to go, not to rewrite it from scratch:

http://talks.golang.org/2014/c2go.slide#18

And, the gccgo compiler does leverage gcc, assuming that counts as a mature compiler:

http://blog.golang.org/gccgo-in-gcc-471

. Neither of these contradict your implicit point of "they chose to invest time in this port instead of trying to maximize performance ASAP", but you still might want to know.