Back in the day when I got my 87 Grand National new off the lot it was sort of a sleeper because word wasn't fully out on the beast and I would surprise many an unsuspecting Chevelle or Iroc-z etc.....today I still have the car but people know what it is. Fast forward to today and I think an ultimate sleeper is a CTS-V Wagon - not many made and they're right fast! If building a sleeper I would probably go with a pickup truck heavily modified as to be stealthy but with radical power.