Is there a way to tune the behavior

Be cognizant of two things that might be impacting your in-cluster behavior: increased cache thrashing and the inherent problem of running concurrent containers on a Node. If you haven't already tried it, you may want to use taints and tolerations to sequester your PG Pods away from other Pods and see if that helps.

what is a good way to measure I/O, etc.

I would expect the same iostat tools one is used to using would work on the Node, since no matter how much kernel namespace trickery is going on, it's still the Linux kernel.

Prometheus (and likely a ton of other such toys) surfaces some I/O specific metrics for containers, and I would presume they are at the scrape granularity, meaning you can increase the scrape frequency, bearing in mind the observation cost impacting your metrics :-(

It appears new docker daemons ship with Prom metrics, although I don't know what version introduced that functionality. There is a separate page discussing the implications of high frequency metric collection. There also appears to be a Prometheus exporter for monitoring arbitrary processes, above and beyond the PostgreSQL specific exporter.

Getting into my opinion, it may be a very reasonable experiment to go head-to-head with ext4 versus a non-traditional FS like xfs. I can't even fathom how much extra production experience has gone into ext4, merely by the virtue of almost every Linux on the planet deploying on it by default. You may have great reasons for using xfs, but I just wanted to ensure you had at least considered that xfs might have performance characteristics that make it problematic in a shared environment like a kubernetes cluster.