I have a neighbor with the megan racing catback and its so loud I hear his car 4 houses down in the basement , I love the sound itsa nice and deep and doesnt sound tinny , But I have been told by him that it didnt make much diff in HP for him. I really dont see the point of spending x amount of dollars on a sound (bass is a different story , jamming is a necessity) ide rather feel the car pull a little harder when I am driving spiritedly I recently did the LSJ downpipe and Injen intake and so far i have noticed a decent difference , not huge but decent , and ide like more where that came from.

so that being said

Which Exhaust is the MOST functional?

 

Piping is piping.. The difference between certain exhausts may be 1-2hp.. Nothing noticeable.. Cat, backs are mostly for sound, yet still necessary to keep the flow from the header/DP

 

just about any aftermarket exhaust will give you the same gains, you just need to pick one that has the sound youre after.

 

Too big of pipes can be bad.
Exhausts flow better when they are hot, hot and hot.
Bigger pipes cool quicker, and something about back pressure...lol.
max on a 2.2 should be 2.5".