Unless you want it to be LOUD to the point of being obnoxious, or plan to race it, I would avoid following in my footsteps with dual 2.5", a H pipe and a single muffler.
I'm considering trying to put a pair of resonators in the area the stock resonators were, versus the trouble of building the system again with smaller pipe.
As it sits on my SEC, it's loud to the point that when it's cold it's almost painful to the ears to be behind the car, and with the slightest bit of load on the engine it has an almost deafening roar.
If I didn't have to live around other people, and didn't worry about my own hearing over the long run, it'd be nice, but...
I'm thinking that dual 2.25" might have been a better choice, but I was being ambitious.
(If I can find other work soon, I might just try to fit twin turbos to help take a little of the noise off rather than shrinking the exhaust... They quiet things, right?
)