What sort of performance impact do these indexes have on writes? I'm guessing PostgreSQL has to look at evaluate each index predicate for the inserted data to see if the index needs to have an entry added.
When we first deployed partial indexes as our primary indexing strategy, we did notice significantly higher CPU. At the time our assumption was that evaluating the partial index predicates was the cause of the high CPU. I later wound up investigating the high CPU use and found the time wasn't spent on evaluating the partial index predicates, but on actually parsing them! I recently wrote up another blog post on how I found this out: https://blog.heapanalytics.com/basic-performance-analysis-sa...