Al, you are using a Y pipe and reducing from 2-1 before the muffler? Ever think that you could benefit from having the exhaust crossing over within the muffler as to act like a crossover (X or H pipe)? I would think that the drastic transition from 2-1 within a Y pipe would cost torque/power/mpg.
I'd be curious to see some testing with your setup vs a dual inlet muffler, know of any?