This is much more easer to do with just simple reducing MTU and MRU on the server to the proper value.
Default change-mss rules are usable in 99% of situations, other 1% is for complex setup, where combination of VPN is used you need something different, so in this case you have to create setup manually.
I am not ready to suffer increase of the CPU load just because every rule will have to lookup one more condition.
I disagree, doing something wrong, really fast, is worse than doing it the right way.
The dynamic mss adjustment rules are wrong and breaks things.
There are plenty of hosts out on the internet that has very low mtu's.
The problem is that mss is set unconditionally and not just lowered when it needs to, thereby breaking tcp connectivity between those hosts.
As an example; If mikrotik.com had mtu of 1400 and your connectivity was set to 1420, you wouldn't be able to surf this site.
I would also like to add that clamp-to-pmtu option will in a typical case only work one way;
packets entering ethernet interface (mtu 1500) and exitting pppoe interface (mtu 1492), then mss will be 1452
but packets entering pppoe interface (mtu 1492) and exitting ethernet interface (mtu 1500), then mss will remain at whatever it was before (typically 1460).
Additionally clamp-to-pmtu works by doing an additional route lookup, which also slows the routers *alot*.
However this is often faster than doing hundreds of dynamic mss adjustment rules (as enabled in ppp profile).
ppp implementations on other platforms often implement mss adjustment as part of the ppp/ip driver and none of the above mentioned drawbacks.