With a transfer cost of course :) Otherwise you could misuse your karma to move your comments up all the while.

And I have a hunch that high ranking comments gain points exponentially, not linearly.

I'm not sure if that's what I mean -- but some posts seem to get more points just because they already have a lot of points.

Like pop songs in that study where they divided people into groups and had them rate songs? It seems almost inevitable. That sort of positive feedback can make a system more sensitive, but it can also make it unstable, amplifying noise until it overwhelms small signals. My third connection is that, if your hypothesis is correct (and I'm pretty sure it is) you would tend to expect comment karma to be distributed according to a Zipf distribution.