I must be missing something. io.MultiWriter returns an io.Writer, so you'd need an interface assertion to take advantage of this, correct? Is there some ReaderFrom-style optimization that I'm not aware of that this would enable?

CL https://golang.org/cl/12485 mentions this issue.

@cespare io.WriteString checks whether the Writer implements WriteString. See http://golang.org/src/io/io.go?s=9776:9831#L268.

@bradfitz that was fast. :) I thought I was going to have more time to poke around and check the rest of the writers, particular net/http's default responseWriter.

net/http's responseWriter already does WriteString:

func (w *response) Write(data []byte) (n int, err error) {
        return w.write(len(data), data, "")
}

func (w *response) WriteString(data string) (n int, err error) {
        return w.write(len(data), nil, data)
}

// either dataB or dataS is non-zero.                                                                                                                                       
func (w *response) write(lenData int, dataB []byte, dataS string) (n int, err error) {