OpenBSD does have a port for the ER series, but it's separate from the SGI MIPS ports:
(I just realized that's a pretty subjective question but I actually am interested in the opinion of someone who has interest in the odd corners of openbsd)
Note: I don't use OpenBSD on it, just the default OS which is a custom roll of Debian.
It's certainly not a problem for me if that's really just a limit on bandwidth, and not some kind of additional CPU load or network latency.
I run a OpenBSD router in a PC Engine Apu1d and it's more than fast enough. I can hit 700+ mbps on iperf with a basic pf rule set. Since OpenBSD is in the middle of updating their network stack to enable SMP I'd bet that number will rise with the next couple releases. I found mailing list posts from a year ago that have the iperf benchmarks of the apu1d right at 500 mbps. I'm interested to see if the network changes that landed in 6.0 will mean a measurable difference. This is entirely academic on a 30 mbps connection but I like it when numbers change.