sorry, my mind was elsewhere at the time. My experience has shown it's not so much poly count, as it is poly density. A close up render of a high concentration of polys in a single location will often cause firefly to simply stop. Hair files are fairly notorious for having exactly that. This is most prevalent in situations where the polys intersect one another, as well. (I discovered the situation while creating plants recently)

thou and I, my friend, can, in the most flunkey world, make, each of us, one non-flunkey, one hero, if we like: that will be two heroes to begin with. (Carlyle)