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. I take it the speed boost you are talking about is what Comcast used to call Power Boost that they implemented in 2006 and eventually licensed to other cable companies? Where you get a temporary increase boot of speed if there is available bandwidth on your node. If so, Comcast and other cable companies stopped using power boost years ago, there's no need for that anymore. Comcast was the first to drop their own technology in 2013.
Comcast to Shelve 'PowerBoost' | Light Reading
If you pay for 100Mb and routinely get 30 Mb, no ISP is going to give you the 'up to' line when you should be getting close to 120 Mb. If you are seeing a third of your advertised speeds there is a problem with or without the non-existant Power Boost.