The SB6121 will do the 50Mbps service just fine, in fact it pull much more than that.
Take a look at the spec sheet...
»www.motorola.com/web/Business/Pr ··· heet.pdf
Theoretical max is 171mbps down and well over 100 up, and you will never see those speeds available. I know guys on the 105mbps service where its available that are getting around 110-120 with that very model of modem.
The problem your having is not your modem, its their end, the modem isn't getting bonded on four downstream channels, without that your never going to see the speed your paying for.