Hard for me to say. I put on headers the same time I supercharged, opened the intake, and opened the exhaust. There WILL be gains I just don't know how much, and it would depend on your other mods.
I have a set of Downey headers I got for my 96' and I'll sell em to ya cheap. I got them because I thought my Edelbrock headers were warped. Turned out to just be a loose fastener so I never put the new Downey's on.
Why yes, yes I do. Interested?
An aside, I'm going to be selling my rig soon. It's got 250k miles, but the ARB winch bumper (winch not included) and the TRD supercharger make it worth the $3000 I'm asking. :D