A real sleeper must be able to blend in the crowd. Park one among other similar cars and no one will be able to tell the boxy looking Volvo that leaves BMW's behind from all the other factory standard Volvos. Install tinted windows, lower the suspension, put sporty tires, a banging stereo, or any non-standard body panel and you don't have a sleeper anymore. A true sleeper must make people go like "WTF
" every time, not just once. It must make people question their own eyes and wonder if what they saw and heard really happened.
For me, a sleeper is like a popcorn salesman who happens to know kung-fu. Just by looking you could never tell that the little gentleman with glasses could beat the crap out of 5 guys who tried to rob him. And even after watching it happen, you still can't believe it.
In order to have a tue, TRUE sleeper, you shouldn't change anything outside, or even wash it properly. The car must always have the crappy looks of an everyday beater. Bonus points for non-working radio, parts held with tape and all kinds of rubbish thrown on the rear seat. In fact some sleepers look to be in WORST shape than their factory standard counterparts.
I don't even consider M5s and Mercs to be sleepers. After you learn how powerful an M5 is, you won't be surprised anymore, and whenever you see one you know it's a mean machine. A shitty-looking old FIAT is a different story, you simply know that they are never fast, so when you see one going past 150MPH it's always a surprise, every single time, not just once.