FYI, I ended up going with the offset/center configuration at the recommendation of my muffler installer. It fits up there very nice and I have a straight shot back from the front to the back bumper and the tail pipe has been relocated to the left (driver's side).
Review of the Borla ProXS Muffler:
The muffler shop tried to talk me into a Flowmaster. He's been brainwashed by someone to think their design is the best. I did a lot of research on my own and even came across a dyno test of all the leading mufflers done on a Mustang several years ago. They compared straight through, stock, and everything in between. By the way, straight through performed worse than most mufflers. The Borla ProXS, which is considered the budget series by Borla, actually outperformed their more expensive models, according the this particular dyno. It also showed that the Flowmaster actually decreased horsepower and torque. That was all I needed... I bought the Borla. I should note that by decision was based on performance and sound volume. I wanted performance without setting off car alarms when I drive by. The Borla is also stainless steel and carries a 1 million mile warranty. Another plus.
I paid about $100 for the muffler. The guy at the muffler shop looked at it and said it looked like a typical straight through muffler, aka "glass pack". He wasn't impressed. After he was finished and started it up the first time, he was surprised at how quiet it was for such a small muffler. Louder than stock, but not the loud, obnoxious volume of most performance mufflers. Just a nice rumble at idle, barely more than stock, but a nicer tone. I then jumped in and headed out to run it through its paces. I immediately noticed the difference from the stock muffler when I took off. Louder, yes, and no more mini van sound. This sounded like I expected a Jeep to sound from the beginning. Again, not a loud hot rod, but more of a performance sound that was very nice - deep, rich, and just enough growl. Something else I immediately noticed, whether it is imagined or not I'm not sure, was it seemed to me that I accelerated with less effort. That I now took off smoother and the engine wasn't working as hard to get up to speed. Again, it might just be imagined, but after about a year, I still think it accelerates faster. Don't get me wrong... I'm not challenging anyone to a race at the stop lights or anything, but I am very pleased with the sound and performance I now have, imagined or real.
Here's a link to the Mustang site that I found the dyno info. Ford Mustang Muffler Comparison Test - 5.0 Mustang & Super Fords Magazine
This is the best article and dyno test I've come across. I hope you find it as interesting as I did.
I also found this link that talks about back pressure. The truth about exhaust backpressure and torque