im gonna have to side with mrgrn here ,i have owned, or had in my possession most if not all full exhausts for the 51 and i can tell you the headers are not where the weight can be lost.
The OEM header makes great slip-on power as well as helping eliminate about all weight loss possible on our bike. the ONLY reason for replacing the OEM header is for the gains that CAN be had with minor mods and tuning to gain mid to top end HP the OEM can't make or for looks for a particular system or mount you might like
I will say the jardine or arrow or akra 2-1 is lighter but all of them do not make the HP or flow as nice as OEM headers
a all ti will do it but not cheap or easy to find. most fulls are lighter overall from OEM but the savings come from the mids and cans not the header sections