Do they?Considering just about all ISPs over provision their speeds by 10% - 20%, you should just about always test over what you pay for, unless you have gigabit.
My VPS shares a 10Gbps Internet connection and when I measure large file transfers (using rsync) over the long term, the rates are much less than the 75Mbps that my plan advertises. The speed tests all rate highly but Comcast is very good at turning out great numbers for speed tests.
You're right about the name of the burst mode being PowerBoost. I must be thinking of another ISP.