What kind of government vehicles are you referring to that are typically modified to legally avoid emission standards? Short of military or emergency vehicles, I’m not aware of This as a practice.
The ones I've seen, IIRC, were from DHS (CBP, etc) or various border operations. They pop up on auction every so often, but I don't have the links off hand.
I know people used to look for retired police cruisers back in the day because they thought they were “detuned” but I’m pretty sure that was a resale marketing myth.
The hardware package is slightly different in PI vs CV along with in some cases a tune. The HP would vary from anywhere from 10 to 20 from Civilian to Police.
Generally though, you'd also want a police vehicle because of how well maintained fleet vehicles are in general, along with some added features and viewing the idle engine hours.