What CARB-EO certified headers are you reinstalling? Most of the dual-cat CARB-compliant headers were not impressive, because of the CARB (and Federal) requirement that the cats can not be relocated from their stock position. As a result the driver side primaries looped up above the valve cover, overheated everything under the hood, and produced little in the way of gains.
There's a CARB website that lists all the CARB approved aftermarket parts like headers, cats, and probably the Y-pipe. There's not much you can do to replace the weight of the cats. So you're left with possibly a thinner gage metal for the Y-pipe tubing. Not a lot there unless you can find someone the uses titanium.
1994 FIREBIRD FORMULA
381ci all-forged stroker (Callies Stealth, Oliver billet rods, BME 2618-T61 nitrous pistons) - 10.8:1 - CNC LT4 heads/intake - Comp Cams solid roller - MoTeC M48 Pro engine management - 8 LS1 coils - 58mm TB - 74#/HR injectors - 300-shot dry nitrous - TH400 - Gear Vendors 0.78:1 O/D - Strange 12-bolt - 4.11 Pro-Street gears - AS&M headers - true duals - Corbeau seat - AutoMeter gauges - roll bar - Spohn suspension - QA1 shocks - a few other odds 'n ends. 800HP / 800lb-ft at the flywheel, on a 300-shot. 11.5 @ 117 MPH straight motor