If you're just talking about allowing e-mail notifications for replies in addition to new topics, which I assume you are, you should check out the hack I did for the UBB5 series. It has a limit on the number of e-mails that will be sent out per thread, so things don't get too out of hand. I use this on my board, which is just shy of 17,000 members, many of which use this feature, and haven't noticed any slowdown. I have a version of this hack working on my UBB6 test board as well, as will release it once UBB6 is out of beta.