Bandwidth needed for the cable modem is insignificant compared to the entire spectrum Comcast uses for their services. If you get 6-9Mb/s download speed, that's still less than a single HD channel. So no, you won't see any degradation. What could cause degradation is splitting your line for your cable modem. If you need to, be sure to use a good quality splitter (1 GHz). An amplifier may be necessary too.