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I have working the tuning industry (mostly subarus) and you need to be careful withyour predictions. The cars brain is the ECU and it tells the car exactly how to run with what it knows. No there is some level of "learning" in these ECUs but not very much when you add mods like these.
We have seen the addition of a Subaru Performance Parts Intake to a stock application and the car LOST 7hp. After the tune it was up 3hp (this is to the wheels). The stock ECU couldn't understand the new MAF readings and all the additional air so it actually performed worse until tuned.
As for cat backs there is really nothing to tune there but in todays cars there is really not much of a gain there since the stock systems are pretty unrestrictive. Maybe 5 hp TOPS. These are mostly for sound.
Now for headers, shorties will do little for a car tuned for them let alone untuned. The real gain in headers is in long tubes (preferably without cats or hi flow cats). This is where the gains are. Shorties slapped on there with no tune will result in very very little. Maybe another 5.
So you may actually net nothing by adding all of these parts without a tune. And no GM will not be tuning your car lol. If you decide to put them on to "get extra warrentied performance" you probably did nothing but make the exhaust note cooler and empty your wallet. Now if you take that car and get it tuned for the mods, it will get you some better gains but now your warrenty will not be honored and they do have ways to detect if you have flashed the ECU.
I am telling you this from first hand experience, never trust a dyno plot of a part from an actual manufaturer. Subaru provides a dyno plot of their intake on an otherwise stock car with 11hp gain. We did the exact same test on the same car and got -7hp. You can make a dyno look anyway you want it is simple drag and drop lol. Only trust dynos from 3rd party tests and real world applications.
My advice. If you are going to mod....go aftermarket and do it right and cheaper. But understand you pay to play. Once you go this route you need to accept that you may get voided on powertrain when you have an issue. It is just the risk you take.
Either stay stock and stay happy or deal with the problems out of pocket if you decide to mod....
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