The Grow method on strings.Builder is documented as:

Grow grows b's capacity, if necessary, to guarantee space for another n bytes. After Grow(n), at least n bytes can be written to b without another allocation. If n is negative, Grow panics.

Currently it grows as such:

// grow copies the buffer to a new, larger buffer so that there are at least n
// bytes of capacity beyond len(b.buf).
func (b *Builder) grow(n int) {
	buf := bytealg.MakeNoZero(2*cap(b.buf) + n)[:len(b.buf)]
	copy(buf, b.buf)
	b.buf = buf
}

I think that it is backwards compatible to start using sizeclasses in the strings.Builder, and not waste memory while growing.