Theoretically, in practice gross emitters are only cited if there are other violations
Oh, for sure. The only time I can think of someone being cited for it was an acquaintance who took his catalytic converters off "for more power" and proceeded to annoy an officer who stopped him for speeding.
What would've been simple speeding ticket ended up netting him 8 fines and a tow truck to impound.
Pro tip: it doesn't matter how you feel about cops, but don't them off. They have guns, handcuffs, and just enough knowledge on the law to be dangerous. You can get your payback in court.