That's sad that you've noted them. Great, but sad. Says he, as he mutters away wondering how to achieve that automatically
The explicit overheads are, I think, 26 bytes per packet (2 PPP, 6 PPPoE and 18 Ethernet). Assume a packet size of somewhere around 1458, that's 1.8% - or 713Kbps of a 40,000Kbps.
Other than that, I think some of the bandwidth goes on an O&M channel between exchange and modem. But I have no idea how much data actually gets transmitted down that.