Yes. If you don't use PPPoE or another tunneling protocol, you can remove the line option mtu_fix 1 in /etc/config/firewall to squeeze out another 200-30Mbps.
Option mtu_fix 1 does not seem to exist in etc/config/firewall
I assume you preserved your settings during recent upgrades? This keeps the old configuration. If you do a full wipe, ie. don't preserve settings during upgrade, it should have that option.
Option mtu_fix 1 does not seem to exist in etc/config/firewall