I'm in Gilbert, AZ with the Cox Premier Package and have a SB6121 and currently getting anywhere from 30mbps to 47mbps. Here's my latest speedtest.net results:
Your 6121 should be able to handle Premier speeds. I was getting better than 60Mb down with my 6120.
Router netgear R6300,Modem motorola sb61416 channel bonding down, 3 upI've gotten as high as 195 down 43 up on different occasions