If you have a modem that runs at 300m currently then you will get no benefit replacing it. If you are going to buy a new modem that is a little harder questions. It depends how many channels they are bonding. The older 6183 is said to support 300m also but only support 16 channels some of the time/warner areas use 24 channels.
Unless the money is a huge issue I would spend the $40 or so and buy the newer model