The ideal is to deliver public IP via BGP, but you can also do 1:1 NAT as a solution palliative and get a system that automates this task.

read: http://wiki.mikrotik.com/wiki/Manual:IP ... :1_mapping

First, never change the MTU of the physical interface! Work with the L2 MTU that it supports! If you are using EOIP to give transparency on your local network, you are suffering unnecessarily! Study a little about MPLS and you will see that your problems will be solved very easily, and does not suff...

If you want to provide transparency to your network, use MPLS/VPLS/TE as well as you do not suffer from overhead you will gain performance with the increase of MTU and decreased fragmentation of data on your network.
Hugs!

http://wiki.mikrotik.com/wiki/Manual:TE_Tunnels_Example