I expereienced the same issue recently, however I woke up one morning to 4 bonded upstream channels. I think cox may be transitioning your market, I waited along time for cox to update our upstream channels in my area, I would read posts here about people not more than 10 minutes from me getting bonded upstream channels and wonder if I would be getting it as well. I think it cox is doing their best to service the areas in need then other areas.
I've been on the ultimate tier for over a year and so far my upstream speeds are pretty consistent since getting the bonded upstream channels. I think it won't be long before you are getting it too.
What tier of service do you have?