They are more or less the same depending on what the department ordered. The engine it self is the same, but some have different gears, exhaust, heavy duty radiator, heavy duty suspension, etc. There is a generally accepted combination of components that make up a "police interceptor" but my understanding is that some departments special order them to meet a specific need or save money on unneeded components (adds up when you are buying 20+ cars), so you won't REALLY know until you start to dig into it. The exhaust adds about 10hp if I recall correctly, and they often put in one step shorter rear gears. This is to compensate for for all of the added weight (police gear, computers, push bar, partner in the passengers seat, lights, dog, crook in the back, etc.). When in patrol trim, the police interceptor isn't any faster than a base model crown vic. It may even be slower with the added weight / drag. Civilian crown vics were also offered with dual exhaust, lower gears, upgraded springs, etc.

With that said, you can mod the heck out of those cars with a little creativity. Most mustang parts can be made to fit (if they don't already) and lower gears, limited slip diffs, suspension, wheels, exhaust, etc are all available. I know a couple folks have even fitted modified mustang supercharger kits on them. Mixing and matching various parts from other Ford products is also a popular way to up the horsepower.