you MUST reprogram the car, what power is the car pushing, the 05+ SC saleen mustangs are just about at their limit on power as they come so be careful~ I wouldnt push more than 400-450rwhp/tq on that engine.
you can put a lower pulley on the car to up the boost like 03/04 cobra guys do~ or you can go with a smaller upper. In the end you MUST have it tuned by a professional that KNOWS that car else you are asking for a rebuild, the 05+ gt engine is strong but still not strong enough to take to much psi.