I run full 3" from the turbo back in my rb25det powered S14.
I did experiment with removing the cat and putting a race magnum in place of it. The exhuast noise did not change dramatically, it did sound a little more uncorked. Locating the muffer under the car where the cat was kept the drone away and left me with a louder sound from the rear muffler (straight through 3.25").
You can "tune" the exhaust by moving the muffler (resonator) under the car to a different location. You can also do this by placing a different length in there.
John, how high were EGT's etc when that muffler collapsed? I am curious because I have one under my car where the cat was.
Evan