First you need to get an intake. Get a short ram, a cold air intake is not worth the extra money and also the risk of hydrolocking your engine. Then you need full exhaust - header, high flow cat, and a 2.25'' minimum size tubing cat-back exhaust. These mods MIGHT make you as fast as a bone stock GSR. I'm not sure because I don't know the gearing of those year GSR's. If they has much shorter gears than a GSR will still burn you.
After intake and exhaust, you might want to chip your ECU. Then get underdrive pulleys and cam gears. Next you might want to upgrade your clutch. Then focus on the intake manifold and throttle body and fuel system. After that get some better cams.
All those mods I listed will probably get you about 150 WHP, way plenty and more than enough to spank even a new GSR.
The clutch is really the weak link in a 2G teg. I had a 90 GS, that clutch slipped so bad with the mods I had I bet if I had a good one I would have run a full second less in the quarter. These are the mods I had on it: SRI/H/HFC/CBE, chipped ECU, fully rebuilt head with P&P + 5 angle competition valve job, crower cams, Nology plug wires, hipo coil, NGK plugs, 17'' motegi wheels, toyo tires, coil-overs, terminator 2 spoiler, red graphics on the side, the car was white. I sold it about a year ago
BIG MISTAKE- that car had potential. I loved it but I needed the money. I was pushing 180 WHP, sucker was fast.