At scale this should work fine. Some parts of the dashboard are not yet prepared for that amount of data to flow in though, may be hard to navigate and monitor.
I am also looking at the pricing for that because right now I offer unlimited emails but I need a good way to compensate for power users like yourself without going completely overboard.
I use sendgrid because it just works, but I'm always curious when a new thing comes around and I'd love to save some money.
How does this work at scale?