+1 on exposing f,__builtins__.

Of course, the thing I'd really want is a way to state that all references to builtins are meant to have the exact semantics of those builtins, so the compiler can translate e.g. len(x) into a new opcode that just calls PyObject_Size().  (I can dream, can't I?)

Another dream: assume that globals that refer to modules, classes or functions don't change, so they can be cached more aggressively.

I suppose enough checking of dict version tags can get us there, or at least close enough.