Im my area of the U.S., one occassionally sees a police car derived from a dedicated high-performance vehicle (such as a ZR-1, Z-28, SRT Challenger, etc), but not often. A radio is worth a thousand horsepower and much safer than high speed pursuits, plus a back-up unit is needed to facilaitate an arrest. These hyper police cars are usually confiscated property as someone previously mentioned, getting as much publicity use as actual service duty and don't last in the fleet all that long. I can only imagine taxpayer outcry for the purchase of, say, a Corvette police car.
Furthermore, new car performance is pretty good across the board. I'd say a local sheriff's Hemi Charger would have little (or less because of added weight) on my wife's Hemi 300C in a balls-out interstate run. Again, though, enter the radio.
Wasn't always that way. Former "police packages" from the manufacturers had an edge on mom-and-pop grocery getters, but now law enforcement options seem to be traditionally limited to severe duty suspension, cooling, high-output alts for the electronics, etc. rather than another horsepower boost. I know the California Highway Patrol and the Michigan State police have their own testing specs that must be met before a bid is secured, but last I heard, performance on acceleration and top speed could be beaten by any 20-something kid with a can of WD-40, a checkbook and the latest Hot Rod Magazine on the dash of his older Chevy, Ford or MoPar.
Interesting side note, though, when I was in Dearborn, MI
,the local PD
always had a (conservative in number) fleet of Lincoln Town Cars, Mustangs, etc. Some marked, some not. SuperFly would love a Town Car with a Boss 302-style hockey stripe on the side "POLICE". The headlight doors would pop up and on go the reds and blues. Ford would lease these vehicles to the city of Dearborn for a few dollars a year, perform all maintenance, etc as they were heavy-usage testing vehicles for new transmissions, rear axles, ac compressors, etc. A win-win (unless you were a perp). Regards...jack