Just ask owners of late-model Ford Mustang SVT Cobras! They were rated higher than they were proven to have, which led to a class-action suit.
One the other hand, what about the new Neon pocket rocket? I can't recall the exact model nomenclature, but I can tell you they are 30-40+HP healthier than the manufacturer claims. This leads to other reasonably fast cars being spanked by those pesky Neons!
Bottom line - I think the best way to figure out true comparable horsepower numbers is to plug 1/4 mile times & trap speeds (and vehicle weight) into one of the online Horsepower calculators. This way, a car's true ability to go fast is baselined, a a legitimate comparison made.
Just my $.02
