A wolf in sheeps clothing is a good description actually.

Something thats looks plain or poo and is disguising a hot engine.

That car looks cool, is obviously highly modified, and is not fast. So its NOT a sleeper.

I think the term 'sleeper' comes from the fact that in a poo or plain but fast car you can catch other drivers in proper sports car 'sleeping' and beat them in races because they are caught unawares or 'sleeping'

Could be wrong though