There more to the multiple channel bonding than speeds. On a single channel modem I would see delays in channels changes that would seem like a loss of internet. I don't see them at all with my doc 3 6121.
tsduke is right.
I was having modem problems with my sb5121 so I got a 6121 and the problem went away. There was an upgrade glitch that was knocking docsis 2 modems offline when D3 went live in september. Mediacombill in the mediacom forums told me D3 was active so I got the D3 modem. They only started selling 50/5 service in our area the last part of DEC. It took them almost 4 months to test and make sure the service was running right.