with the header and larger exhaust piping on there, you are losing some of your backpressure. once exhaust gasses start building up, at around 3500 in your cases (they begin to increase sooner than that obviously) more back pressure is created giving you the power. on my old 5th gen, it was really obvious when i took off my header and exhaust when returning the car to stock. this was on an f22, so its not that much different than the f23. now i have a 6th gen with the f23. its by no means "torquey", but low end doesnt seem too bad, but then again, everything is stock for the time being. hopefully that explains the situation