As well as a completely disproportionate number of poisonous snakes, so it's a tradeoff.
If there had ever been a formula for measuring the success of a sleeper, it would probably have something to do with the gap between your expectations of a given car's performance and the reality of it. In that case, this incredibly boring-looking Toyota Starlet from Australia is the king of all sleepers. It's got a 1.5-liter four-cylinder engine under the hood, but this has been turbocharged up to 440 horsepower, and now it runs 10s.
We've seen cars which claim to be sleepers that have actually been ratted to the point where they still stick out, but this car is just completely unassuming, it's fantastic.