On your stock car, you won't see much in the way of peak power increase by replacing the stock headers. The rest of the stock exhaust system - especially cats and mufflers - can be improved upon. But unless you get that stock motor up into the 280+ HP range, the stock HO headers won't hold you back.
Now, if you put on a set of small primary longtubes (1.5" primaries) with a good system, you can really perk up the bottom end and mid-range. But most power gains on a stock car are gonna come from the system downstream of the headers. Putting on equal or non-equal shorties are a waste of money for you unless you're planning on other significant changes in the near future.
Having tmoss port your stock lower intake will also really help - that component is the biggest single flow restriction on the stock car.