Get the intake of your choice, a vette-air, a good tune, and maybe a ported intake/throttle body and you will be more than satisfied with out the headers. Now, if you want to do a cam, the headers would be advisable although some have had great results with cams and the stock exhaust manifolds. The stock manifolds are just that good on this car so the headers don't add very much.
I have American Racing Headers, a friend has LG headers. A tune and intake mods are much better bang for the buck. The long tubes will give you more mid range torque, but not much more peak HP or TQ. Maybe with a cam, don't know.....YET