Wouldn't a simple fix to this problem be that crit bonuses above a certain level of crit start to increase the critical effect component rather than the actual crit chance?
I mean at the moment they have this ridiculous dichotomy between high ilvl fire mages doing reasonably well and everyone else not lucky enough to have been smashing 15/15 HM fed up with the RNG associated with their crit chance. If they just left CM alone in the first place (1.5), put in some kind of crit cap (e.g. 50%) so that higher ilvl crit bonuses start to add to critical effect damage.. I dunno i just can't help but feel this would solve a lot of problems.
I don't know how much it should add to the critical effect, but that seems like a pretty easy thing for the devs to work out from a balance point of view.