Great question, I've asked myself the same thing.
First, in my opinion they serve to achieve different things. While openwrt is a firewall, it'd a simple zone based firewall and it designed primarily as router firmware, not firewall software.
Opnsense is BSD based, openwrt is Linux based. Those both haves pros and cons. BSD has serious pedigree in the networking world. Juniper switches are still based on BSD even. Openwrt gets the Linux traffic shaping goodies like cake though.
I chose openwrt because it's more suited to my environment, where I have 10 VLANs, a 10G fiber core, and want IDS/IPS. Openwrt is meant to be lighter weight, but is less feature-full.
I would love to if I had them! Haha. I'm working on the dashboard right now, which will be part two.
I don't have a great answer on the IOPS requirement, but I imagine it's less than something based on elasticsearch/open search based on the reindexing. I'll try and benchmark it if possible.