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Interior is of course all preference.
Intake - I wouldn't bother at this point. Not enough of a benefit without more mods. Most of which being a tune. This is not a car from 20 years ago with an inefficient intake. There is a good chance your fuel mileage would go down. Maybe just get a drop in filter.
Exhaust - Q300 is the probably the least loud/obnoxious, followed by Perrin, but there are many others that sound good without being obnoxious. Most of any quality are going to be over your $700 though. You also are not going to see much in the way of increased efficiency.
Sway Bars - Do you know why you want sway bars? I see no reason to change sway bars unless you are planning more suspension changes, which you would probably want to do first anyway, such as springs(under 700) or coilovers.
Underbody Panels - nice to have, but there are more useful things. The extra efficiency would never pay for itself in the time you have the car.
Really the best way to increase efficiency in this car is to drive it efficiently. It has low rolling resistance tires, and super light oil from the factory so they could put as efficient of numbers on the sticker as possible.
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