I understand that when looking a your broadband's speed you need to take
into account various protocol overheads.

However, if I measure my broadband download speed directly at my modem using
SNMP would I still need to account for this overhead or is it included in
value I would see?

If it makes any difference I am using Paessler PRTG monitoring a SNMP sensor
on my Motorola SurfBOARD SB5100 cable modem.