Yeah, even "proven" gains can be a can of poo too!
I bought my car with a crucial bit of paper in the service history which is a printout from the engine tuners that rebuilt the engine about 25,000 miles ago. It said that on an engine dyno, the engine made 171.3bhp with a direct at the fly measurement. That was damn good and I used to quote that figure all the time when I first bought the car.
But when I finally got the chance to strap the car to a rolling road, the power figure didnt add up. OK, so it was the first time the car had been used in three months and the exhaust was blowing like hell, but Id be lucky if the 128bhp the car made ATW equaled more than 155bhp at the fly. It was the best 16V on the day and I guess its near enough to make excuses, but theres no point. There were also a couple of much more lightly modified 16Vs in the ballpark of the power figure too (benJ - grrrr!).
Theres no doubt that the car has the mods, but either they dont add the power they claim or the engine has "lost" 15bhp since the last dyno (a dodgy one?). Either way, I take power gains with much more of a pinch of salt now.