I think all Comcast DOCSIS 3.0 is rated for 50 down. I get around 40-50 down and 7-10 up on their DOCSIS 3.0. I would call and ask them about it if you are paying for the 50 Mb service...
mine must be low cause i have the cheap package of comcast cable. its fast enough. i rarely get alot of lag on my wii when i play call of duty modern warfare reflex.
Keep in mind late in the night your internet speeds will get better due to powerboost. Comcast allows users using the internet to siphon off others who are idle and not being used to get higher speeds when needed.