Why are you running 89 octane in your car? You do realize these cars are suppose to run on premium? That is probably the reason why your car runs like shit. You need to start putting in 93 octane and stop being cheap, especially if you are boosting. You are fucking your motor up badly and your are causing detonation, so start filling up with 93. You need to run all that shitty 89 out and refill with 93.

As far as your gas mileage goes, there could be several reasons why you are getting shitty mileage. You timing could be off. You may have boost leaks. Your car is not tuned and could be running too rich. The reasons go on and on.

And as far as your leaks go, you need to pressure wash the engine bay to clean it from all the oil and do the same for the underside of the car. Then, try to locate all your leaks.