I have a mate that works at a ford dealership where a few cop cars are contracted to have there work done on, he said that the ecu simply has a 'cop mode' that you can switch too when using the diagnostic tool and that all it does is takes away the limiter and raises the idle a bit, he isnt sure if it gives any more power or changes anything else. Also they weigh heaps because not only do they have the computers and radio and siren crap but the boot is usually full of equipment, gun/ammo riot gear etc, which is why they are usually below legal limit in height at the rear.
As for weird cop cars ive seen a few new marked magnas, ive been pulled over by an old crappy magna, i saw the smart car that was funny and my mate was pulled over by an unmarked bmw.