Well first off, cross-flow mufflers are "somewhat" restrictive. Common sense tells us that, the sharp turn required to get through one causes backpressure.
And we ALL should know that less backpressure ALWAYS means more power, forget the myths that you need backpressure to make power...it's a myth.
What backpressure affects is torque, similar to the way headers do. And you can tune it for drivability on the street, or lower the peak torque rpm and broaden torque curve.
I personally love the way duals sound(..to each his own..)since the early days. And if I can squeeze out a few more ponies in the process, well....
I personally dropped 3 tenths from a popular 2.5" cat-back going to 2.5"duals even with the muffler shop bends.(which I'm improving now..)I guess you could make a similar comparison, like 4-valves to 2-valves per cylinder. Even smaller valves in pair will out-flow 1-big valve....

 

But can she type?

Oh sorry, thinking of a porno movie I saw.

Anyway, the 2.5 in is probably stock and yes, there are gains from that so going with a 2.5 dual exhaust helps. No arguement here. But can the 2.5 true duals go up against a 3 in SLP or a 4 in Mufflex and still show gains? That is the qustion.

Unless you are making some serious power, I wouldn't think that the 3 in would make the car outperform the 4 in. After all, if more flow is unnecessary, would it help?

I did a few test runs then I unbolted the exhaust past the headers and before the "Y" once and did a few test runs. The first runs were with a test pipe so that the cat is not a consideration. The runs were not significantly different. Of course the weight was constant because I had kept the exhaust pieces in the car.

YMMV

 

orrrrr, run 3 inch OVAL duals....

and remember bigger pipe only means less velocity, which means decreased low and mid range pull


personally i think a 4inch exhaust with 3 inch tail pipes would sound jussst right especially with a nasty cam