JFTDWS
+++ Out of Cheese Error +++
I honestly think 14stone- and I am assuming you dont mean 14stone rider + tack - is too heavy for ANY lightweight 15hh horse, cob or not.
It may not be your intention, but the inclusion of "15hh" in your description of the inappropriate mount could be taken to imply that a larger horse may be more suitable. I would suggest that a LW cob-build 17hh horse may well be less suited to carrying 14st than its 15hh counterpart. I think the issues with % style "rules" is that it feed a general misconception that bigger is better, or that a heavy draught build horse may be an appropriate weight carrier (many really, really aren't).
Fwiw, I've had 14odd stone on both my highlands - very different riders, different circumstances, but I deemed both to be acceptable for them at the time.