Yes, and very. I'm running an SB6183 for the Extreme 150 package (currently the fastest in our area, but a little birdy told me that 250mbps will be deployed in the next few months over here).
The biggest advantage of having a modem capable of more channels is not necessarily the maximum theoretical speed, but having the additional channels allows the network to load balance your traffic to your modem. For instance, if four or five channels are being heavily used by your neighbors, being able to go onto others will give you more consistent speeds. (This is assuming your node isn't completely over subscribed, the additional channels are available in your area, and the server on the other side is capable of maxing out your connection. And also, this is Comcast, so this is also assuming they're not going to be major A-holes and artificially throttle you).
As per my internet package, I'm supposed to get 150mbps down, 20mbps up. In my area with this modem on good quality RG6 cabling, I see about 185-188mbps down sustained, and 25-26mbps up, sustained.