Not many people believe in an aftermarket chip boosting your performance by whatever percentage the anufacturer describes. In most cases, the chip causes power loss not gain.
In most cases, these chips were developed with a specific car in mind. I'm not talking about a make and odel, but the manufacturer actually grabbed a Probe or MX6 and strapped it to a dyno and what not and played with the specific settings for the chip in order to make it perform at it's highest potential. Now, if you grab that same car, once the chip was made for it, and added headers, high flow cat and exhaust per say, that will through the chip way off. Then, the air/fuel ration and other specifications will be thrown off and the chip will make the car loose power instead of gain power.
In my opinion, they are a waste of money. Unless, you can have a custom chip made to work with the modifications you have. This means have your car dynoes and tested as they make the chip.... Only problem after this is that if you add another modification, you'll have to reprogram the chip.
My .02c