Hey Bob, can't understand why you stayed with single exhaust and large muffler seen from back.
I understand the thing about exhaust mods. The xfire needed some aggressive sports car sound and I found the answer.
I converted to full dual cat-back system with similar round stainless tips out back. The difference is that I eliminated muffler and replaced with much cleaner lookin two small free flow converters. All you see is the dual pipes coming out back. I agree that the low-end power is about the same but once you get into 2nd/3rd at 3grand and up, the power comes on. I haven't taken any measurements but sure seems more powerful.
It stands to reason that if you can free up the exhaust and find a freer flowing intake at the front, ya got improved performance.

For those who might be interesed, I'm going to try to get some pics and upload.