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Hard question to answer as it depends on which ones ie. 1 5/8 vs 1 3/4" , the intake that you have as well as the carb and flow rate of your heads. I have been reading the posts about this for a few years now and if I have learned anything it is that performance gain is a never ending search for more, and that you really have to sit down with a pen and paper and put the whole puzzle together within the budget that you can afford.
There is no definite answer. Anything you do to IMPROVE breathing will yield more efficiency and more horsepower. How much....do it and see. Also, all headers aren't the same. If they are not made correctly, you could have some problems. The tubing should be equal length and the diameter should be within certain measurements. Too big....or too small isn't a "good thing". Also, be careful on the muffler you select. If you add a set of great headers and a crummy overly restrictive muffler...you've wasted your time and money. Use a K&N air filter while you're at it and you will be surprised at how much bang you got for your buck.