Ey guys I was wondering if anyone had done something like this before? I have some 2009 gt cutoff mufflers that I was considering swapping out for my stock v6 ones I have a 2011 v6 will it make the exhaust sound louder? Does increase in hp or mpg? Any info would be great thanks!!

 

It'll probably make it a little deeper, but not really much louder. Not really a difference in HP or MPG either. It's just for sound.

 

Anything can be made to work, but what you are asking might not be very simple. The Axle-back exhaust interface changed after 2011 on both the V6 and GT. After the 2011 MY they all use a ball & socket type interface between the after-cat pipes and the axle-back. If you wanted to use a stock 2005-2010 GT setup on your 2011 V6 setup, you will have to modify the ends of the cat-back pipes to accept a slip-on coupler like the earlier models used. LIke I said, it can be done, you will just pay the muffler shop a bit more for "custom" modifications to make it work for you.
As far as sound/performance/mpg's... I really cannot tell you because as far as I have seen, there are not many people doing this. It will sound different, that is for sure.
I am currently using a set of 2010 GT take-off axle backs on my 2010 V6 which I converted to dual exhaust. IMHO it sounds great (for a 4.0)