I've been using the RRS L-plate on M9 for several months, and from my experience much prefer it to the Kirk plate. The only slight annoyance if you use a camera strap is that one end of the strap attaches to the camera and the other end to the plate so the camera and plate are always kinda married. But it works fine with Arca products including the cube and the P0 and Z1 ballheads; also with RRS monopod head. It obviously won't work with the optional Leica-supplied M9 handgrip -- no bother to me because I've found a Thumbs-Up is a more secure grip anyway.
Don't buy a Kirk or Arca-Swiss plate that merely screws into the existing M9 bottom plate!! If you knock the camera accidentally, or even set it down in a hurry while attached to a monopod or tripod the supplied bottom plate is liable to twist off the camera sideways, leaving it inoperable. I know that from experience ... and I'd recommend the RRS ...
David