Forget improved mileage, you just won't see it.
If you want a power bump for a good price I would suggest the stock exhaust from the Nismo Z. It has a better sound, larger pipes, nice tips and perfect fit. Clean used ones should run you $400 on the private party "parts for sale" section of this forum. This exhaust is well built from stainless steel so it will last longer than the car. It is worth maybe 8-12 horsepower, not as much as some of the aftermarket units but good bang for the buck.
As for intakes, I went with the Stillen G3 long tube intake. It is CARB 50 state legal which means it is certified to meet or exceed factory emissions specs and is good for about 18 horsepower. It is beautifully made out of polished aluminum. My dealer has been fine with it for the last 2.5 years.
The two together should get you to 350+ horsepower at the flywheel for a total of about $1,000 if you do the install yourself.
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2010 Brilliant Silver, Base w/Sport Package, AT7, Katzkin leather, upgraded audio and lots of Dynamat and a bunch of small stuff.
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