There is not any major improvement over true dual exhaust and a well built single setup. Before you all flame me re read what I said... WELL BUILT not just throw a pipe behind the cat and put on a good muff. Get better cats, and have the exhaust built from the headers back and you can achieve the same performance with 1/2 the pipes and also keep your car from scraping or rubbing the driveline. These cars are not really engineered for true duals but you can make a really nice sounding Z or SS with about a grand in the pipework under the car.

Massey

 

Massey is right. In fact, in a properly sized exhaust system, true duals will make less power and a narrower torque curve than an x pipe or a single pipe. This is due to the same scavenging effect that makes equal length primary headers superior. You'll also save an easy 20 pounds of dead weight

 

X pipes and H pipes also promote scavenging. A 2.5" true dual setup on an fbody will outflow a 3" y pipe one and will make more power.

 

Edit: I apologize, I thought I saw 2 inch, not 2.5. You're right, 2 2.5 inch pipes create a flow area of 9.7 inē. One 3 inch pipe will create a flow area of 7.06 inē. However, one 3.5 inch pipe has a flow area of 9.6 inē, a negligible difference from 2x2.5 with significant weight savings
Two schools of thought. I'll concede that true duals sound nicer in some situations. To me its not worth the weight, when a single pipe can have 95% of the tone depth
I also mentioned an x pipe. Leave h pipes to the silly fords