How good is the Go optimizer? I would guess that there's still a lot of room for improvement because that's not something you'd spend a lot of time on early in the life of a language.
People interested in generating more optimized machine code (more slowly) should check out gccgo, which is a GCC frontend for Go, rather than the default (fast) gc compiler suite.
Basically, not much (depending on which compiler chain you are using). The Plan9 based compilers (which compile at light speed) don't produce great code.
They don't produce great code because they are optimized for compilation speed. IIRC, the plan is to have your quick to compile compiler and your fast execution time compiler..
It's not the plan; they already exist: gc is the default, fast suite of compilers; gccgo is a Go front-end for GCC.
I should have been clear: it is the plan to maintain this dichotomy