My personal experience is that there is far less difference between an alloy and carbon stem compared to other components both in terms of weight and feel.
I am a new GR300 owner, but bought it as a full bike from Lynskey. Once I have the fit dialed I will be putting either a Lynskey ti stem or a Thomson alloy (to match my seat post). Atheistics are my biggest consideration on this build when it comes to the stem.