I work in our county jail and our road patrol has them, they arent that fast...maybe high 15's in the 1/4. I believe the interceptor package is just a handling package and some heavy duty engine parts, no real power mods.
My dad worked road patrol for years and always said the LT1 caprices are faster....
i just remeber my dad saying back in the day, cough, that interceptors came with nice bigblocks in them, and he was chased down by one, cop clocked him at like 148 in a Comet w/ 302 BOSS, racing some porsche, but that the cop said he didnt have much to catch up with them, but for a that big of a car, i knew that it had to be beefed up some...