Looking for info on upgrades 1998 Mustang GT

Hey guys, i've got an 05 mustang, but a friend of mine has a 1998 GT and I told him i'd ask around for info on upgrades.

He wants to make more horsepower and was thinking of a few mods already but didn't know what he'd gain. He was thinking of...

CAI
Custom Tune
Underdrive Pulleys
Exhaust
and Headers

So what are some ball park numbers for those mods?

Thank you guys