This seems like a very good idea in terms of controlling spikes in comments - but you have to question whether this is the (main) cause of the degradation. If

This seems like a very good idea in terms of controlling spikes in comments - but you have to question whether this is the (main) cause of the degradation. If limiting new accounts seems like the best solution, it should be done clearly and transparently - as the users you are trying to weed out are generally fickle and will not stay, so won't bother to check / sign up later. Let users know the reason why they can't sign up now, and when they should be able to. However, users that have a legitimate need to comment (eg site / app owners) need to be allowed access - solving this could be trickier.