Hi Jason. Adjusting the MTU is a good point. You can define the MTU on the router, and just "ip tcp adjust-mss" so that it will implicitely force the clients to lower their MTU.
the mss will be 40B less than IP MTU (as you substract 20B for tcp header, and 20B for the additional IP Header that is used for tunneling).
you can test for example a MTU of 1400 and an MSS of 1360.
The mss is the maximum segment size. if the router adjust the mss, it will rewrite it during the client connexions so that a TCP segment can't be more than 1360, thus implicitely lowering the MTU on the end stations.
Hope this help.
Regards,
Bastien