»ww2.cox.com/residential/palosver ··· _premier
shows Premier as 25/2 (31/3 with powerboost), Ultimate 50/5 (55/5.5) At the moment, if you are hitting your max speed with 4 channels down, there is no a reason to upgrade the modem.
If Irvine, CA mirror's AZ's speed upgrade; Premier should be 50/10 (63/13 powerboost) and Ultimate 150/20 (188/25). If this were to occur and you are on Premier with an old 100mbit router, the router may become the bottleneck. 4 channels down can support 63megabits down, but may not during primetime. Obviously you will need a 8x4 modem and gigabit router for Ultimate.
Price-wise, you can pickup a 8x4 Cisco DPC3010 used modem on ebay for the low $30's and sell the SB6120 for a cheap upgrade. I am not sure of the benefits of spending the extra money for a 8x4 SB6141/SB6180 vs the DPC3010.