On all cars w/ any type of aftermarket panel filter made of cotton/gauze/paper etc., dont expect too much. You may get an slight increase in MPG, maybe. Throttle response should be better. As for HP gains, there can actually be a decrease in HP, I have seen that before believe it or not, but the gain or loss can not be felt. The main purpose of an aftermarket panel filter (especially on a NA 4 cylinder) is for reuse.
I noticed an improvement (albeit minor) in how the engine pulled at higher RPM's. Nothing drastic but throttle response was def. improved. Mind you, I have a WRX and I don't if you would feel it as much on an NA car
My 09 2.5i is much better off with the stock subaru/denso filter. I had my new avo highflow in for about 4 days and experienced very noticeable and significantly diminished throttle response in 1st through 3rd, slower acceleration, and consistent bogging with near stalls in first with spirited takeoffs. After the install and driving for a couple days, I took the filter out, checked the fitment and reinstalled just to be sure. Nothing. Once the stock filter was back in it, my 2.5's fun, dependable response was back instantly.
So...any wrx owners want to buy a near-new (~30 miles) high-flow avo (S1B03G4VB00) for $20?