Since we're talking about a fuel injected engine, it becomes a lot simpler process to take care of what you're doing. I'm not familiar with the computer system in cars, so i don't know if it will automatically adjust the fuel delivery based on sensor readings. So that would be the first bit of info you should seek out, and i don't know where you can get that info. If the computer does not automatically adjust, then you need to find out exactly how to adjust it. Sometimes this can be done by changing out the chip in the computer, other times you can download a new injection program and load it into the stock computer. This info you will have to get from someone else, as i don't know.

As far as hp gains, you are not going to notice a thing unless you start talking about 15hp more. Some people will go and change their exhaust and swear their car feels more powerful, but that's not always 100% true. You can, for example, put on an exhaust and claim you feel more power from it. then you have to evaluate WHY. does the engine feel more powerful at 4K rpm now because it's actually producing more power, or is it because there's now a severe lack of power at 3K rpm and now you feel a "boost" at 4K rpm because you are finally making power again. to REALLY assess what your engine is doing, you will need to take it to a performance shop that has a dyno. They can run your car on the dyno and use the proper analyzers to measure your exhaust to see if you are running rich or lean, and they will also be able to check your hp and tq curves to see what your engine is REALLY doing and where you are/are not making power. My ultimate goal when entering a situation like this is not to increase hp and tq at a certain rpm, but to make an improvement across the ENTIRE rev range and to smooth it all out. To do all this the proper way, it's going to cost you quite a bit of $$$. My suggestion is to contact performance shops in your area and find out how much all the proper testing is going to cost and to see if they really know what they are doing. You will do better in the long run if it's all set up right and tuned perfectly. Otherwise, you are just wasting your time and money for no reason.

Also, don't base your exhaust choice by what the manufacturer CLAIMS you'll make in hp and tq gains. you need to see proven results. and everything must be taken into account for all of this. your altitude will play a role in this as well, because as your air density changes, your engine will require a different amount of fuel to compensate. Hope all this helps!