I think mine qualifies.
My 540/6 was also something of a sleeper; single exhaust, "yeah, it's some BMW..."
This guy was pretty sleepy barring the wheel-filling brakes.
320whp STi swap.
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2002 Acura RSX » 2001 Impreza 2.5RS » 1996 BMW 328is M-Sport » 2000 Impreza 2.5RSTi » 1999 Mustang GT » 2002 Lexus IS300
I love threads like this where someone asks to see sleepers, and then a bunch of guys post cars that (albeit fast) look heavily modified, or obviously modified to be fast.
I'm pretty sure "sleeper" refers to a car that is generally thought of as slow. A car that has been made to be fast yet still looks slow.