I don't have the Stillen CAI, I have the Takeda one, which is similar, and I didn't notice any drop. I can almost average 30 mpg on the highway still.
Installing my CAI was my first mod ever, and I had to go out to buy a wrench set because I had nothing but an adjustable wrench which was unable to get the bolts at the bracing. Of all the engine mods to do, this'd be the easiest. You could always ask a friend for help and tools. Then again, if you actually feel uneasy working on your car, then definitely take it too a shop. You'd be kicking yourself pretty hard if you end up scratching your bumper or braking a MAF sensor just to save a hundred buks or so.
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