A sleeper in my opinion can have a hard cam rumble and exhaust noise and wheels and a cowl hood etc. just not a show car. Looking like a dog turd helps. What makes it a sleeper in my opinion is when it drives the streets often, it looks like 12s, but it really does 9s due to a power adder that can’t be seen while driving. Building a sleeper that only runs 12s or 13s is just asking for embarrassment from every kid with a mustang or camaro. Unless you can keep it ‘perfectly stock’ quiet. Ain’t gonna happen.
Again just my opinion.