How come your chosing an a12 over the a15. The a15 will come on boost sooner and have heaps more torque which is really what your seeking. The boost will increase torque more than it will horsepower on a setup like this. All i can say is go for torque. Especially in a slightly heavier sunny wags. It will make a world of difference to driveabilty let alone performance. And if you dont want to risk breaking an a15 go for an a14. Cheap as chips.

Oh. And go an SU for self tuning abilty. Nothing but a needle to change. Plus you could get an 1 3/4" for less then $20.