I particularly like the sound that more lenses are coming. I'm actually not dissatisfied with my M body for the minimal amount that I have invested in it, but I really want a EF-M mount telephoto along with another fast prime somewhere around 50mm.
In the meantime, have you considered the EF-M to EF/EF-S adapter?
I own it. Have you considered the size of the 70-300L mounted on an M body? I'm not trying to be snarky, but my point is that I would like a very compact (M sized) telephoto to take for a tele option while traveling. For general purpose travel I often carry my 70-300L along with a general purpose zoom (Tamron 24-70 VC) and one of my 6D bodies, but I shoot relatively few tele shots and that makes the 70-300L a weight that I sometimes regret carrying. A compact tele on the M would satisfy that need while taking up far less space and weighing a fraction of the 70-300L's bulk. The lenses that have been developed for the M have been excellent, and a compact 50-200 tele would be a great option.