It does not seem to have any substantial effect. Individuals who are 'moderately' withered require 20+ withering reduction before they even fall back to Lightly afflicted. And this is from a level 7 cleric.
It seems something is wrong here, and if it isn't, I'd make the suggestion that the XP sacrifice that the PC clerics make in regard to reducing the withering of the faithful be either lessened or the rate of their removal increased 2 fold.
Last I checked it was 4-6 remove curses to get someone from moderate to light, this does seem very extreme.
Try 20, from a level 8. Just went through it last night. Even had GSF Abjuration. Remove Curse is an abjuration spell. Who knows if it actually has a hidden effect on it or not though. The gap between third stage and fourth stage (moderate to heavy) isn't even as long as the first two (touched, and lightly afflicted).
Tried to bring Norgen's out of Moderate with about a dozen or more remove curses. Gave up. Still moderate. Something else is going on and I am sure the answer is the dreaded FOIG. :)
Not a bug.