It most likely doesn't, you may just be imagining it in most places in the power curve. The resonator is a neutral device in some locations in the curve, like any musical instrument they're tuned for a very specific range... the weak spots.. that's where the most noise is made and as such where performance takes the biggest hit. In some systems this hit is minimal due a more efficient design of the rest of the system, staged throttle bodies and so on. There are also very different types of noise removal devices... some really are only for making things quieter and some do cause a small level of restriction... not the case here.
Here, read through the following link... last time I'm searching info for others. There is a TON of information about this topic by people far more qualified than garage motor heads who half drunkenly assume that added noise = more power.
http://www.supramania.com/forums/sho...ight=helmholtz
K&N makes VERY poor filters by the way (this too is available on the net
)... let's also cut to the chase on a couple other things while we're at it: larger diameter intake AND exhaust piping are not always a good idea (ESPECIALLY on a N/A); and a larger air filter that's sitting in a hot engine bay, sucking up air that's 20 degrees or more above the outside air temp causes BIG losses in power. You don't see a huge change in mileage because you're actually getting LESS air.
I'm done. Find your answers and think what you like it's obvious I'm not going to convince you and you're definitely not convincing me.