Looking at the specs for the ModbusTCP protocol, I don't see how you can implement it without modifying the code in the ME. Since the ModbusTCP works by embedding extra codes in the TCP protocol, it would need to have specific support. If you are somehow kludging it in there externally (prebuilding the packets?), that may not be an issue, but you are still limited by the speed of the micro.
Are you using SLIP or PPP to get into the Digi? Otherwise I don't see how you are connecting to it on the serial side.