The Rusty's air intake saves everthing except the big rectangular air filter housing box over the passenger side fender. The box is replaced by an extremely simple, tubular K&N cone filter-type mount-thing. The (relatively expensive) K&N air intake replacement system replaces all the intake parts right up to the throttle body- and I don't know how much different the Rusty's is vs. the K&N. I'd say the diff. is probably nominal. I'm happy with the Rusty's.
In the end, I think there's only so much you can do to the 2.5L. I've seen guys add superchargers and intercoolers (see 4-Bangers United, http://www.forumco.com/jeffy/
), but at that point, you'd be better of finding a vehicle with an 8 already in it.