dont bother with the NA tune up, since you cannot have a tune up and a turbo on at once. your best bet is to get the turbo (either stage 3 or 4, what ever the best for the mustang is, i dont have one yet so i dont know) and if that isnt enough for the GT series then work on the lancer. you dont need pure power to beat the last race, you need power and control. a car that can go fast but have crap handling is only good for the test track, if you balance your car properly you can easily win the last race. my lancer did it with a top speed of 202 and 581 HP.