[sperry]

Larger diameter headers on N/A cars usually doesn't help for power unless the original headers are grossly undersized. Usually aftermarket headers on N/A cars make gains by tuning the length to properly match the speed of sound in the tube coupled with the exhaust pulse spacing to allow the sound waves to help extract more exhaust from the cylinders.

Frequently, if you go to a larger diameter header pipe, you do flow more which gives you some more top-end, but it's at the loss of low-end torque. I'd rather keep the low-end that I use everyday than gain a little horsepower that I would only use when wringing the car out to red-line. YMMV w/ the 2.5i though... I don't know how the 2.5L N/A motors react... but I would bet ideal performance would come with much longer primary pipes of stock diameter, and keeping the equal length. But that would sound crappy, and be hard to fit under the car.

If sound is the goal, some stock diameter UEL headers would probably work best. How hard is it to convert a pre-08 header-back exhaust to work on the 08s?