I use OBSD exclusively on my gateway devices. Currently using it at home on an HP730 with a quad-port GB Intel NIC. Total cost ~$200 (used)

Keep you eyes open for sales. /r/homelabsales/ has great deals on these SFF devices sometimes.

I've read that the PCEngines struggles to keep up with network traffic in excess of 100Mb/s.

Although this update may have addressed that. That router guide is ok. but the official pages are better. https://www.openbsd.org/faq/pf/example1.html

Hit me up directly if you have any questions. o2kewl[at]gmail[dot]com