maybe its because of the exhaust?
i've read somewhere that if you increase the diameter of the pipes too much, it would have a negative effect on performance.
increasing the diameter by a balanced amount results in less resistance but a little less velocity (of air inside the pipes) thus more flow. when you increase it excessively, the resistance decreases too much, so there isnt enough pressure inside the exhaust, so the velocity becomes too slow so then you have worse flow.
i know this is not very clear but hope its a bit understandable??
maybe rap's lack of torque has something to do with this, i dont know though.