I'm one of those "changed to headers but did not change the Y-pipe and change to the 3" tubing", and I'm one of those that report no significant changes. So like said earlier, I can only report my results. Given the post about results with headers/Y-pipe/3" tubing, sounds like he has a lot more experience with the 4.7V8 than I have experienced.

What I do know of engines is all 4-strokes come down to breathing, and breathing at the right RPM range to realize real performance result. Headers, exhausts, have much less to do with sound and everything to do with creating power at the right RPM range for gains the driver can feel and use. If the changes make more power, at higher RPM, and its a RPM range you don't use, no gains. If the changes make more power, especially more torque where a driver can really use the gains, that's worth the efforts. The 4.7, even the 4.7HO in stock form has a sagging torque curve with a power peak at about 3,300 RPM. If the right changes flatten out that sagging torque curve or even make it a humped torque curve shape, thats gains a driver can use and feel.