said by NetFixer:If you know a way that the customer can put an SMCD3G-CCR into bridge mode, you would probably gain a few new friends in this forum if you shared your knowledge. 
No I don't sadly. I however know some users in meatspace with SB6120's in on business class service.
My point was though that the ARP traffic is totally normal for cable modems so it wouldn't change your WAN speeds.
However, generally people don't use proxy arp for their LANs. I'd argue it's a minor security issue in fact. But it will not effect speeds.
