Are you certain that a user's karma factors into comment sorting? It's clear that more recent comments will be sorted to the top (temporarily), but I definitely haven't gotten the impression that existing karma scores affect anything. I don't know the factors of the real algorithm.
Edit: This could be related (someone please correct me if I'm wrong), comments are allowed some time at the top before they "mature."
(let mature (table)
(def delayed (i)
(and (no (mature i!id))
(acomment i)
(or (< (item-age i) (min max-delay* (uvar i!by delay)))
(do (set (mature i!id))
nil)))))