At this point the code using FMN is getting rewritten and any additional notifications will need to be in that rewrite. That said, IRC is actually one of the fastest ones we can push to. Each additional service adds in slowdowns as the communication requires accounts, special routers, people who actually use those services to know how they work, and various other infrastructure with the only return being a ton of complaints when there is any problem in them. 

Every person who uses notifications is an endpoint who has to be 'polled' by that notification, and there can be a lot of overhead in doing so even when people think of it as a fire-and-forget-bus. Every failed or slow or bad end-point slows down the entire system which grows over time. Email delivery problems from mailing lists are generally caused by the N hundred developers who decided they needed every notification as an email and have clogged up the outgoing queues because every mail system decides that a mass rebuild is a spam attack. The same goes with other systems (IRC, etc) where a large number of notifications do not look much different from a DOS. 

While notifications is a great idea, it is also a hard problem. Please help the team who will be working on it.