I just found this nice tutorial about the v6 tunnels and the ip over-head in particular.
http://ardenpackeer.com/routing-protocols/tutorial-ipv6-tunnels-part-1-manual-gre-ipv6ip-tunnels/
http://ardenpackeer.com/routing-protocols/tutorial-ipv6-tunnels-part-1-manual-gre-ipv6ip-tunnels/