hi james - i've heard all this before and i will give you a few tips. first, it depends what you are wanting to do with the car. if it just an everyday driver, that's one thing, running it on the strip, that's another.
here's some bare bone facts. at sea level, there's 14.7lb psi pushing down on the filter surface. when the engine draws it's charge for the intake stroke, it has only that 14.7lb to push air into the cylinder, so naturally, any restriction on the filter surface is going to result in a pressure drop with less pressure at the cylinder intake. if you were to compare a fresh stock air filter against a k & n at idle, there should not be any different. it would be much like a gentle breeze blowing through a screened window. now, as soon as start pulling higher rpm's on it such as a race situation, then that screen is going to whistle and so it goes with the stock filter vs the k & n filter. the k & n filter will shine at this point.
so, what do we know. we know that manufactures like to twist facts. it is a fact that at higher revs, you can get better horsepower out of a low restriction filter, but to advertise it like it does it all the time is not right, but that's the art of selling.
if you want to get extra horsepower fairly cheap, change over to synthetic oil and you should gain about a 3% increase in horsepower due to the reduction in internal friction. so, with a 200 hp engine, that would be a net gain of 6 hp. not bad for the money. and this is a gain that you have available all the time, not just on top end.
hope this helps.
~ curtis
87 GT
89 GT conv
90 GT
93 LX 5.0