Looking at getting a Magnaflow muffler only for by Cobalt S/C. I have been reading posts debating that 2.5 in is the best for keeping your power, and 3 in. is just over kill. Anyone run both? And does the size only matter going into the muffler or the tip as well? Dont want to change the performance negatively. And could you also explain your answer as to why you go one way or the other.

 

So your only changing the muffler?

In reality if you have stock exhaust other then the muffler the muffler itself won't change much except for sound.
If you were doing full exhaust that would be a different story. 2.5 is more then sufficient for anything the stock supercharger can manage bigger would just cause torque loss due to back pressure being lower. Unless you were turbo swapping in the future or getting a tvs. Even the.. 2.5 wouldn't become restrictive until well over 300hp

 

go with a 5"

 

Please search, this topic has been discussed a million times over.