No, that is the
maximum.
(Ethernet is limited to 1500 byte packets. See
http://en.wikipedia.org/wiki/Maximum_transmission_unit for details.)
MTU is not generally considered a "tuning" knob. In general, one reduces it from 1500 downward, in Ethernet networks, in order to manage special purpose network configurations, such as PPPoE.
For more guidance on OpenBSD network knobs, please read
www.openbsd.net/faq/faq6.html#Tuning