I ended up going with the Leopold Deltapoint because I couldn't find a mount that mated the Fastfire III with the XDm (they have them for XD but not XDm.)
The XDm now has Bar Sto match barrel fitted by Scott Springer, extended mag release, stippling, trigger job, solid stainless guide rod, and the Deltapoint (all by Scott Springer.) Mounting the Deltapoint was a real cake-walk - simple and quick.