I would probably go with exhaust and intake mods first. Maybe ignition as well. I do not know what ignition system your particular car comes with but upgrading those will give more power from the engine.
Rear end gears really depend on what you want out of the car. IE 1/4 mile car or top speed car. Basically the lower the gear ratio the slower the top speed will be and you also have to tune new cars when you change gear ratios so the speedo is correct.
As for LSD I would definitely go up to Posi-Trac for better traction but that can always wait. I would do engine and exhaust first to increase HP. Cat removal will increase HP by quite a bit but is illegal in most states.
No muffler or high flow mufflers will increase it a little.
This guy kinda sold you some bum dope.
A rear gear swap won't necessarily decrease your top speed and it will only decrease your 1/4 mile time by fractions of a second. However, being a turbo car a steeper "numerically larger" might help you get into "boost" faster since you'll redline quicker (at a lower mph) in each gear. Also limited slip and posi trac are the same thing. Posi is what GM called it.
Having no catalytic converter will help a little but not enough to feel and mufflers won't make any extra power, just sound.
Ignition components also don't add any extra horsepower. Zip, zero, zilch, none. Old cars (older than the 90's) needed ignition upgrades to be able to support "extra power" that was made. Your stock ignition will handle more power than you'll probably ever make. "High performance" ignition coil packs and such are just gimmicks.
Cold air intakes won't help much if any and and aftermarket mid pipes (X pipe, Y pipe, etc) will help a tad but maybe not enough to notice.
If I were you I'd look into a good tune and up the psi that the turbo is making. More psi is really the only way you'll notice any more power.
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