LadyBuickGS, I use K&N filter on all my cars right down to the minivan we use for dog shows.
They will make it easier for the engine to get air into the combustion chambers and this can increase HP as well as mpg.
The thing to keep in mind is that with most stock engines there isn't that much HP to be had by just slapping in a free flowing filter.
Think of an engine as nothing more than a large air pump. Its sole purpose is to suck air in and push it out. The easier it can do this as the more it can get in the better it will perform.
If you are considering a K&N it won't hurt the engine and it will not void the warranty on the car. Just keep in mind if you have the stock exhaust on the car that is the other side of the air in-out process. You might want to look into an aftermarket exhaust to help that GS breath better.
The Maxima SE had 0-6 and 1/4 mille times much worse than the GS. Not bad compared to an SUV but not great compared to what I consider a performance car.
I like the GS a lot.
Check out Turbobuick.com and go to the 3800 supercharged Tech section. It is all devoted to your car.