Well, obviously there is no magic bullet. What I don't want to do is waste money. What I have learned, through working on my other car, is that different engines respond very differently to commonly available modifications.
Possibly of interest, I added an Apexi Versa Cruise grounding kit to my Corolla and, according to my local dynojet averaging three pulls, I added about 3 ft/lbs torque and 3 hp to a low-torque high rpm engine.
Now, clearly the 2.4 is a different engine and doesn't redline at 8200 rpms. It is, however a fairly high-compression engine. The dyno charts I've seen for intakes show a dip in power in the mid-band. That's not acceptable to me. What I'm considering is a dry high-flow drop in filter for the stock box with a stainless exhaust—largely because rust is such an issue for me.
I've not yet seen any real evidence that the stock box is particularly restrictive, but that may not be the case.
Beyond that, thinner tires and possibly a more efficient ignition would be the end of this phase, before I start looking at the suspension, audio etc.
I'm looking up IPF now. Thanks for the tip.