He wouldnt even gain 10-13hp at the flywheel, let alone at the wheels. Most of the gains you feel from the header, are all in your head. You think you put a performance part on, so your a little heavier on the gas. Its just how it goes. But he definitely didnt get 10-13hp from just a header, trust me.
Anyway OP, I skimmed through the first two pages. Im assuming your car is automatic because it has a remote start? As long as you like your car, dont worry what others say. Just do yourself a favor and look around at what others have. You may like your car now, but there may be some other things you might like better. Its always best to keep an eye on your options. And enjoy the gas mileage. Probably best to do some routine maintenance on it right now, since its a little older. Anyway, enjoy the site