I have been running an Injen CAI for over 10 years, still feels great. I even bought it used in great condition. Save for cleaning the filter every year, it is fire and forget for me. For exhaust, instead of going for the ebay lone 4-1 header( did have it before), went for the 90's 626's 4-2-1 header. You need to invert the hangers and modify the connection for the egr bung on the header, but any muffler shop worth it's salt can do it, no problem. It worked so much better for me since the 4-2-1 design mates better with US spec's power delivery. I also run a 2.25 muffler shop made exhaust( should have gone 2.5 given the muffler), and a Racing Beat/Mazdaspeed axle back. Sounds beefy, yet fancy. You dont really need injectors, it comes pretty rich on its own given what little you can do to it