i'm pretty sure the velocity through the 45's wont be high enough to have them running at there peak performance, On your A15. Torque performance has a good reputation for dyno tuning, as per the boyracer video "HighOctane". I think 2-2.25 inch is far better to run, If you run too big you loose back pressure which gives you mid range torque, the dyno people would be able to recommend the correct size from their readings