I still have that magazine.. and that's 400hp to the crank..not the wheels. You can take a stock 5.0 engine and put it on an engine dyno and get on 250hp..but when you install that same engine in the car it will maybe have 210hp to the wheels.
Alot of manufacturers claims are correct, or close to correct. Just like many of you said, some of them are on ideal engines and under ideal curcumstances. The other thing to take into account is where the car makes those gains in the power/tourque curve. Alot of times they are not at max RPMs. A set of mufflers may add, just as an example, 14HP, but that may be 14HP at an RPM no where near max power. The example motor may may max power at 5000RPM, but the mufflers added 14HP at 3300 RPM. If you put the motor on a dyno and only look at the max power gain, you may see only 5HP at the 5000RPM. You would have to look somewhere else to find anything close to the manufacturer's claims. But I do agree, most the claims are based on ideal motors and ideal conditions, with the best dyno run from their tests groups.