I'd say a sleeper is any car that is unexpectedly fast. Doesn't have to be Lamborghini fast. Just faster than is should be.
And sports cars could be sleepers too. If you have, lets say...a C4 vette that can slaughter a Ferrari Enzo, but it looks like a stock Vette....that would be a sleeper.
Or if you have a Yugo that does mid 14's. That time might be slow to you, but it's faster than you'd expect a Yugo to be, so that's a sleeper too.

Enough talk. Here's another:
1929 Ford Model-A with 250 HP Cosworth Rally Engine!
250 HP 2.0-liter, 4-cylinder powertrain from a Ford Escort BDA Cosworth Group 4 Rally Car! The rally-powered Model-A can go from 0 to 62 mph in just 5,0 sec while it achieves a top speed of 124 mph.
http://carscoop.blogspot.com/2008/01...-cosworth.html