IIRC OpenBSD's version of pf is single-threaded, which might be an issue depending on your network speed and hardware. A single core of an Atom or Celeron might struggle on a gigabit or 10gig network, if that's the use case.
I might have the history wrong on this one but it sounds like FreeBSD forked OpenBSD's PF code at one point to add SMP support. OpenBSD's continued PF updates have not been merged into FreeBSD due to the incompatibilities introduced by their SMP changes. I believe single thread performance has also increased quite a bit since the fork happened. I don't require 10gig support currently but I have no doubt that SMP support will eventually be required if OpenBSD wants to remain usable as a router in the future.
My externally facing firewall at home is an OpenBSD box with an Atom C2550 @ 2.40GHz, it's plenty enough to handle all internet traffic (internal network traffic doesn't go through it). I don't have 10gig internet link at home though (who does?)