That get's into our indexing strategy which Dan talks about in this talk[0]. Currently our tables aren't completely insert only, so a BRIN index wouldn't work for us, as one row in the wrong place can cause a huge amount of extra reads.
No more question :-) Thanks Michael for you time, and for the link to Dan's talk. It's great to see how, at Heap, you scaled a multitenant SaaS based on PostgreSQL!
[0] https://www.youtube.com/watch?v=NVl9_6J1G60