the 6121 was not on the approved list here in nyc which is why many were stuck getting the more expensive one. it's likely the number of channels varies by regional system, and is not a nationwide limit on twc which has to compete with Verizon FiOS here in the northeast.
Same here on the west cost the SB6162 is not on the approved list but TW still will activate it and support it, I just called and gave them my new MAC address and it was activated with in 5 mins. Thought this might help
yes, we've heard several anecdotes of not listed modems getting activated anyway - kinda sucks that TWCable doesn't just publish the whole list, but my cynical guess is Motorola paid them not to so they could move the most profitable and most channel stocked inventory.