Can anyone tell me the horse power gain and mpg status with K&N Stock Replacement Air Filters vs. performance one?

 

Negligible? The factory intake system is pretty decent and without other major supporting mods changing the filter won't get you much.

 

^ +1. you won't see any noticeable difference with either hp or mpg. the k&n is not as much a performance mod as it is a maintenance mod. it's still a good idea since the filter lasts forever when cleaned properly...

 

like Canucklehead said, it's more of a maintenance mod. I don't need to buy a ~$15 filter every 15k miles. Just wash it, I think, every 50k miles.
Every once in a while, I use a toothpick to take out some debris and feather like crap and rotate it inboard/outboard and reinstall it back. The center air duct pushes the air mostly on the outboard half of the filter.

 

The K&N filter advantage, like someone said, is minor. The place where folks get into trouble is in the filter's longer service interval, which often means that some car owners may ignore it until it's clogged up and affecting your mpg. The K&N are environmentally friendly (not much of an issue for G owners who like their power) and will pay for itself by 2 cleanings.

 

combine it with the z-tube and you will get a great sound! i dont have any plans on getting the jwt pop charger... stillen or what ever else... the factory intake is actually pretty good... and will have less heat soak than the open cone filters or metal cold air intakes