I've done some benchmarks over the years and 6-7k/s for getting simple data out of a basic Postgres installation seems pretty spot on. The question is when using Postgres as a cache, are you taking away performance for actual, more complex business logic queries. That is going to depend on the level of overlap between endpoints that use caching and those that need more complex queries.

What I'd be interested to see is a benchmark that mixes lots of dumb cache queries with typically more complex business logic queries to see how much Postgres performance tanks during highly concurrent load.