Way too much of that written by the sales department.
"superchargers actually add horsepower as a percentage gain (percentage of an atmosphere)"
Except for the power lost in the drive, pumping loss, and heat expansion of the charge.
"if a supercharger gives your engine 14.7 psi of boost (another atmosphere) that will essentially double the output of your engine"
This never happens.

If you're actually interested: http://victorylibrary.com/mopar/blower/blower.htm