When I took the snorkel off the air box, w/ the K&N filter I had since new, it seemed to have more power on the highway in overdrive (w/ my oversized tires)....pulling up hills better.
I also got much better highway mileage....the only other thing i did was put some Gumout 'Regane' fuel system cleaner, and then Marvel Mystery Oil in the gas & oil.
(oops....I did have the wheels balanced and the front end aligned)
I am not sure what worked best, but what always took about a third of a tank now takes less that a quarter!
I am VERY happy....and hope it lasts.
Next I am going to put on a free flowing muffler....should be a little more power, and mileage.
Every little bit helps w/ the 4 bangers.
p.s. My 1998 2.5 has a 'tuned' factory cast iron exhaust manifold....it looks like 2-1 headers.