Would be interesting to define the phrase true duals as I feel most people do not really know what a true dual system is.
A true dual system is not two exhaust manifolds,comeing into a Y, than catback, you still have one pipe out, if you go from there you may have two muffler systems, but only a set of headers ( two independant systems for each side of the V type engine, passing gas thru two separate cat and muffler systems can be called true duals)
My first catback system was two flowmasters out, looks nice but is show only.
With the 4.0 SOHC engine, the biggest H.P gain will be with a single 2.5 inch system, catback.this was proven in my case with two Dyno runs thru six pulls
What most modders miss is, they see twice pipes, gotta be good, engine can breath better, NOT, if that wisdom was true, than fab up a plenum and run six flowmasters catback, the more pipe,the more bends, The more mufflers, = more back pressor, plain and simple, JMHO