IF I WERE UPGRADING...
Best power upgrades usually for the money are (a) performance air filter/air box and (b) performance computer/chip.
After that, you might move into a less restrictive exhaust - downpipes, high flow catalytic, high flow mufflers.
Personally, as a non-mechanic, I would be hesitant to start monkeying with precision-engineered components such as cam shafts, rockers, or new carbs (though most cars since early/mid-80s are fuel injected). This affects the entire system, and if the car has a computer, these changes might require custom reprogramming to get optimal performance. And this work typically voids warranty, if you have one.
After doing mods (a) and (b) above, the next most important to me would be (c) transmission cooler and (d) shift (improver) kit. Then I'd get a drop-in high flow exhaust.
Of course, you can't stop there. You need to improve handling with new coils/struts, wider/better tires, beefier stabilization bars/arms, etc.
PS - and I had an 87 T/A 5.0 for much longer than I should have, and it was a fun car to drive. Heavy, not too fast, but lots of fun. DO NOT get an 86 or 87 unless you have a guarantee that it had been repainted correctly. These years are notorious for paint problems - when GM switched to water based paint. Didn't acknowledge it until long after my warranty was up.