Relying on speed tests such as those was your first mistake. Take nothing tests like those say as the truth. Go download a huge file (1GB or more works) from a known fast server. That will outlast the power boost feature and when it evens off, it'll show your true speed.
Depending on what you pay for, it sounds like you have Blast! Which is either 25/4 (27.5/4.4 sustained) or 50/10/15 (55/11/16.5 sustained) depending on your area.
See here as well for more info: »Comcast High Speed Internet FAQ
»What are the actual Provisioned Speeds?