well in reality you probably do have 260 at the crank. i would think that the k&n would be worth around 2-5 maybe 7hp. the numbers you are looking at are rear wheel horsepower numbers. the horsepower numbers in brochures are horsepower measured at the flywheel/flexplate not the rear wheels. you are probably putting down somewhere from 215-225 hp at the wheels.
No, our GT's arent underrated like the 03 mach 1's and 03 cobra's. Just like bdcardinal said, we have 260 to the flywheel and between 220-235 RWHP (rear wheel horsepower) depends on if your car is auto or stick. Youll usually see autos around 225RWHP and sticks a little higher, but not much. Just incase you dont understand why theres a difference between flywheel hp and rwhp, when the power is being transferred from the engine through the tranny, driveshaft ect. you lose some hp. Thats not a very detailed explanation, but you probably get the idea.