Hello Tom,
the IP overhead depends on your choice for the payload size.
The emulated circuit is sampled and the samples become the payload of an UDP/IP packet in blocks decided by the payload size parameters.
if using payload size = 512 byte
the UDP/IP packet will be 512+20+8 bytes
if you use 40 bytes of payload the overhead effect is much bigger
78 bytes towards 40 bytes and this will take almost 3 Mbps on wire.
Payload Size
Payload size is the number of bytes put into each IP packet. This parameter impacts packetization delay and efficiency. Configure a high payload size to increase packetization delay and efficiency. A smaller payload size reduces packetization delay and efficiency.
see
http://www.cisco.com/en/US/docs/ios/12_4/interface/configuration/guide/hbbfeamo.html#wp1049052
you can decrease the required BW if you use payload compression or to double it if you choice to use data protection.
of course it is wise to build the CEM tunnel using loopbacks interfaces' ip addresses.
Once you make your choices you can provide QoS resources on the ip path to the CEM tunnel.
Hope to help
Giuseppe