The only difference between a cop car and a regular car is its cooling system, electrical system, and depending on the model, the suspension.
Cop cars have bigger radiators and fans to keep them cool in long pursuits and while idling for hours. They have beefier electricals to run all the lights and computer equipment.
They dont have any special performance engines or exhaust or chips or any of that crap. Its just a car with some extra stuff on it, everything is functional.
I havent driven one, nor have i seen one clocked, but chances are they are slower than stock, because of the extra weight and strain on the motor from running all the extra stuff.
HOW DO I KNOW?
Be on a volunteer fire department in a small town, you get to know everything about public works vehicals. I know all the cops and firemen in the town, so you see stuff and talk to other cops and firemen from different towns.