They might do so once they roll out 24x8 bonding/modems or add to the/bump 305/65 plan. I wouldn't consider 305/65 as Comcast responding to the current 300 FiOS speeds because it's not easily available for most.
Also, where do you see 505/100 ? All I see is 150/65.
This and any Comcast response would be a fiber install like whfsdude 's CC isn't going to immediately upgrade ALL DoCSIS equipment again for the few actual customers this would draw from their nationwide footprint. These speeds and attached prices are still a very limited speciality market at this time.