The whitelist for Whispers and IRC are the same. That’s existed since Nov of 2015-ish? Well before my time. 
You can apply for it via Support. Not everyone gets whitelisted though. We evaluate each bot on a case-by-case basis. It’s also just an elevated limit for messages, so you’d still have guard rails to work inside of.
Re: rate limits. In general, we approach rate limits from an abuse perspective. E.g. Hitting the REST API with 1,000 req/second or sending thousands of messages/second to IRC. You’ll get a polite-but-firm email from me in those situations. 