If your car still has stock unported heads you will actually be loosing torque and power both. Believe it or not the stock header is better when the heads are stock and unported. I had my car dynoed here at a local shop with stock headers then with 1 1/2" equal length MAC's then with 1 5/8" FRPP shorties. All with a VRS x-pipe and Dynomax cat back with 2 1/4" Mandrel Bent tails. I actually lost 7 hp and 6 ft. lbs. of torque with 1 5/8" and lost 5 hp and 4 ft. lbs with the 1 1/2" equal lengths.

Then just to make sure I put the stockers back on and the power went right back up.
You will definately pick up power if you go with the MAC Prochamber and cat back but with stock headers. If you plan on doing head work and intake in the future and then bigger headers are the way to go. But I would wait until you do the work to put them on.
I hope this helps you. Just some things I experienced first hand.