Established L2TP tunnel between two Mikrotiks. Ping with packets 1500 from one to another without tunnel going without loss. Also ping with 1450 packets through L2TP tunnel (MTU/MRU 1450) going without loss. Was delighted to learn about the MRRU feature and tried to set MRRU to 1614 for both sides. After that begins packet loss (for packets bigger then 1440 is 100% loss). Then reduces the MTU on the L2TP server and the client to 1440 - ping started but for packets bigger then 1430 with more than 50% of losses and more, for packets smallet then 1430 no loss. It even does not matter whether fragment flag is set or not. Tried to disable encryption and compression - no difference. ROS v.6.22. With what it can be related? An interesting feature for me, but unfortunately I can not get her performance. Thank you.
2017 year but we still have only one relatively fast type of tunnel (workin on UDP) and with mtu problems (MRRU not working as expected so needed dancing with TCP MSS) 
Do you understand what these options do and why you generally can not set them to arbitrary values?
Having to use smaller MTU on tunneled packets is normal. TCP MSS clamping solves most of the issues,
proper ICMP handling should do the rest. Of course you should not block ICMP “fragmentation needed”
messages in your firewall!