A sleeper to me, would maybe have the V6 with a TRD SC or a V8 with a different front grill that didn't have the V8 on it, but with a TRD SC. That would be a sleeper. When i had my mustangs or Lightning, they would have walked all over a v6 or v8 unless they where SC'd. If they were SC'd and hung with me, i would of said, that's a sleeper. But for normal, not really. Maybe from 4Runner to 4Runner maybe. Have a V8 and remove the emblem, then have a V6 race you thinking you have a V6 not V8, but the V8 is only minimal faster.
To me, as an example, A car i saw while stationed at Tyndall, florida. Mustang LX, it had multicolored/primered panels, 2 rear rims where the same, but two front where totally different, bascially 3 different style rims. Looked like a POS but it could run mid/high 11's. That's a sleeper.
If i wouldnt' of known and it pulled up to me while i was driving my 95 Mustang GT, pushing 300rwhp, i would have raced him and lost, unless he sucked as a driver.