Okay, so here it is 2 years later than my post above and here's an update (or two).
In 2011, after purchasing my '11 in October 2010, I asked a shop to put on a Y-pipe, add MagnaFlow 70's, and then mount my stock tips. What I got was a T, a pair of 40's (too loud!!!), 90° bends at the inlets, the mufflers being visible from the back, and the tips directly welded to the mufflers. Sounded great only at full throttle - anything less was just too much drone. His "fix"? - Cut the part of the pipe that goes around the gas tank and put in a glass pack. I was in a hurry since I was leaving for a long trip in a few days.
Needless to say the trip netted me some great mileage numbers, but at the cost of my ears. On the return trip (we had 2 drivers going over, just me driving coming back) I was running 75-85 all day long in the mountains of WV, VA, and KY and got 29.5 mpg!
On returning to the shop and asking to have the 40's replaced with something quieter I still got "regular" mufflers with 90° bends at the inlets and had to toss the stock tips. Glass pack was still in.
A year later I went to the guy who did my '09 (should've gone to him first!) and had him do what he did to my '09. He pulled the glass pack out and put in a pipe, put in a Y where the T was, made 45° bends to the round muffler/resonators, and then installed new chrome tips similar to my stock ones, except these have a Bowtie embossed in them. Nice lookin', too! Me happy!
There is a little more drone than I'd like but not enough to worry about. At 75 mph I get about 27-29 mpg and at a cruise-controlled 60 I have gotten over 34 mpg. (Search for my posts where I lay out the details if you want.) At 65 I still get over 30 mpg, and this is on a car with a 3.6L V6!
When the first attempt was removed in favor of what I have now it was revealed just how poor of a welder the glass pack shop was. There was a hole on the top where the pipe did a 90° at the inlet and it was about the size of a quarter! Big ol' glob of weld all over that never sealed the hole. I was blowing exhaust straight up onto the floor of my trunk. Glad it's gone!
If I had it to do all over again I would first see what just removing the front resonator would do. It's huge and right in the middle of the tunnel. Heck, I might do that anyhow just to know. (I'm curious as all git-out and just can't seem to stop sometimes!
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If I find out anything I'll post it somewhere on this forum, either here in this thread or in a new one.
So, the guy whose welds were not that pretty is the guy who made the second effort on my '11 and who I'll go back to again. At least his stuff works!