My OEM center resonator (muffler) is starting to blister, I had the shop look at it and they said its just the outer shell that has rusted and I probably could get a year out of it. The muffler is still quiet...

I did get a quote on having something installed that looked like a small cat for $100 They would just cut out the resonator and install this piece in its place...

Would a setup like that make me loose MPG or performance? Would the car be noticeably louder?

 

no on the loss of mpgs or performance, yes on being louder - but not as much as with an NA car since the turbo quiets the exhaust somewhat. Then again, you may like the new tone (there's some vids on Youtube where people recorded their cars with a straight pipe)