5.0 mustang and fast fords did a bolt on test for a 99+ 3.8 mustang. I have the issue somewhere, but can't seem to find it.
If I remember correctly though, the true dual exhaust system gave about 6-8rwhp.
I wouldn't go the single exhaust route, simply because it just doesn't look as cool
2002 Mineral grey GT 5 speed. Flowmaster 40's catback, Mach 1 grille delete, timing advance, push button starter, Bullitt pedals, FR500 steering wheel, K&N FIPK, Self ported upper intake w/ spacer, Pioneer Avic D3, MGW shifter, MM FL subframe connectors, Mach 1 springs, Bilstein HD struts/springs