John,
I did a similar experiment back in 2003 as Hib. I used a Magnaflow 2 1/2 inch x-pipe and the Magnaflow Corvette C4 3 inch mufflers. I wanted a 3 inch x-pipe but they did not make it at the time. I talked with Magnaflow and got them (or got on the bandwagon) to produce a 3 inch x-pipe and put it in later. I did not dyno the 3 inch x-pipe but felt the car lost some low end.
Here is the result of six pulls each: 1) stock car 2) stock car until the cats then a 3 inch piped system with a 2 1/2 inch x-pipe and 3 inch mufflers.
The resonance was tough. Slightly worse than a B&B with x-pipe.
Ted