Previously I noted in the mod_qos discussion group some odd testing results for this feature. I didn't receive any advice.
As I noted in a recent follow-up (still stuck in moderation???), I've worked up an alternate implementation for limiting output bandwidth for requests matching a certain event. That can be found at github/trawick/tweak-qos. I have of course solicited advice from the author and offered my changes under the same license as the original.