From what I've heard and seen expect 10-30rwhp from headers. 1 5/8 shorties being at the bottom end of the scale and 1 7/8 Lt's being at the top.

Here's a dyno run DYNOHe gained 14rwhp with only 1 5/8 shorties and 2 1/2" y-pipe

 

HEADERS
they have some decient prices on SLP headers on this page

 

IMO the best headers out there for our cars are the Jet-Hot LTs. I've heard FLPs are nice but they are damn pricy.

 

by the time u buy macs and get them actually ceramic coated, you coulda paid a GP price for JetHots with emissions...