Really? Are you sure about that decision? If it were my site, I'd decide the other way, if only because I'd worry about the interaction between high-karma commenters that can rapid-fire respond and normal users who have to wait.
Unfortunate, and I say that as a user with more than 10k karma. All comments should earn their endorsements on their own. High karma users are as capable of mean/stupid comments as anyone.
Given that karma is a function of votes from articles posted as well as net votes from comments made, is there a way to use the second but not the first for karma limits related to comments?