I have an IPSec site to Site VPN between a 2805 and an 1841. Both have fixed IP's but the 1841 end uses an ADSL connection and PPPoE. The MTU that comes up on the Dialer0 interface is 1454.
I can get small packets through the Tunnel no problem (standard pings) but larger packets do not pass.
You can apply it on both. Below is an URL that explain the MTU Issue and option in detail.
** Please rate if it helps **