I haven't done a detailed comparison, but going how the OpenBSD guys go with other re-implementations (e.g. LibreSSL vs OpenSSL), I'm going to guess that it will have a reduced feature-set and capabilities.
I've nothing against the OpenBSD guys, but sometimes its a case of "right tool for the job". OpenBSD is great for firewalls and routers. Of course OpenSSH is without competitor, its just awesome.
But (some of) the rest of the OpenBSD toolset I'm not too sure ... for example Chrony is lightyears ahead of OpenNTPD etc.
Postfix is an excellent MTA. Written from a security-first mentality. Its widely deployed, and hence battle tested. As a result of its wide deployment its pretty much a case of "everyone knows how it works", the Postfix mailing list is excellent in terms of getting support.
TL;DR personally I see no reason to replace Postfix with anything, be it OpenSMTPD or anything else.