demostrated nor is it supported. As a matter of fact, on the JuniperSRX and Fortinet FWs, you can't adjust mss within the gui or cmdline for ipv6 tcp traffic. Also ipv6 uses a different approach with mss setting and controls and prefer fragmentation at the client's ends.
And in the above two fwpolicies, you have to configure the fortigate cmdline only. So get use to editing via " config firewall policy" :)
There you have it, my list of various ways to adjust TCP mss adjustment for traffic.
Freelance Network/Security Engineer
kfelix ---at---hyperfeed --dot--com