I wouldn't be surprised if Comcast could effectively deliver 150mbps to all subscribers on their network and just uses the excess capacity to provide wifi. Of course that sucks if you paid for the full 150mbps. ;)
Of course, that doesn't mean where their network connects to the internet won't get more congested...
For cable, the other wifi network is served on a different docsis channel, and shouldn't affect your speed - at least not the part from your router to the other end of the cable.
Well, the other end should be a Comcast central office, if they don't have the necessary bandwidth backhaul, you have at least a very valid reason to complain.
Of course, that doesn't mean where their network connects to the internet won't get more congested...