Most of our complexity is in the sync later, so we spend more time tuning our Python sync server. For example, we wrote a custom statistical profiler: https://nylas.com/blog/performance
I'm currently in doubt to move to either Mailpile or your platform on my personal server. However, PostgreSQL would be a big plus for your platform, since I don't have any experience with MySQL in recent years, and I rather do not run it on my VPS just for mail.
It's not something we're likely to support officially in the near future since we don't run PostgreSQL and don't have the ability to test it well with production loads, but it'd be neat to see a working community fork.