Why not just make 1 the ceiling karma for all comments across the board? Or, in other words, why not get rid of karma all together?
I guess I'm still confused by how your proposal will solve anything.
It seems pretty clear that those people who really are commenting in good faith and aren't doing so to accumulate karma won't be swayed by whatever karma they may get (positive or negative).
So what does explicitly limiting the karma those good-faith commenters get achieve?