Here's the problem. Most think that when they go to a 2.5" pipe that they are just increasing pipe diameter by a small amount, but in actuality that's not the case. #1. The factory does not use mandrel bent pipes which means that in some areas the pipe diameter is actually reduced to about 1.5". You take the smallest diameter within the system, and that's basically your max flow. We've cut away some of the suspected factory bends, and got the measurement of 1.5". This is not to say that some will be bigger and some will be smaller, as they are not all the same, but that gives you an idea of how it works.
In most all out high performance cases, bigger is better, but you do need to understand that your g6 is not an all out racer. By that I mean, that it has a certain perameter that it needs to work within in order to pass emissions and offer you the drivability you expect from your daily driver. The other problem is, if you increase your hp in the higher rpms, you generally decrease your torque in the lower rpms, this is why it's important to control the back pressure. Hp is not what makes your car quicker, it's torque. The average G6 is a 3500 lbs. plus car, that is powered by a V6. V6's generally do not put out that much torque, so on a 3500 plus lbs. car, losing any amount of torque is really a bad idea. Having more hp in the un-usable rpms ends up cheating you big time, because you have lost the most important thing (torque) in the rpms you needed the most, which is down low. Torque is what you feel pushing you forward. I can explain how our exhaust corrects the issue even with mandrel bent 2.5" pipes, but I would be accused of trying to sell a system, which I am not. Those that want to know, just have to read what I have written in the past. For the OP, you can regain some (not all) of that lost torque by simply installing a smaller diameter pipe somewhere within your system.