K&N and a turbo back exhaust would be the best way to go. There is no 02 in my wife's Mazda6 intake, the computer just senses the increase in air volume and air intake temp and it richens the mix (I think) and turns the CEL on unless you get the CP-E intake. That thing is worth 22 peak fwhp and I think 16 average, but it's $600.