Sleeper is in my opinion, when a car looks like nothing spectacular to the naked eye. so the average motorist would.t notice it, but a truly trained eye may be able to find a few hints. Hood scoops, spoilers ,lips and the like, are an instant disqualification 'cause any 10 year old boy will see them and say "that car is fast" and that's not what sleeper is. Even having really nice paint, and a fresh wax implies something other than sleeper because it shows that you care for the car and are putting time into it.
My car is almost there, I have a 2002TS that has a WRX engine running 14 lbs of boost, and full LGT drive line. It has been broadsided, which really adds to the sleeper look... I am working on covering up the FMIC. I opted for the FMIC, because it is much harder to see than a hood scoop, and when meshed over properly, it can be almost invisible.
P.S. All of this is IMHO. IF you'd like to argue a point, that is fine, but please don't flame