The stock tranny will hold stock power just fine. It will handle bolt-ons no problem. For how long? That's a crap shoot. Installing the trans-go shift kit, and adjusting the PCM tables slightly does wonders for it's life. There are guys doing that were GM Serta replacements, and spraying nitrous THROUGH the shifts. Last time I talked to him, he had 50,000 miles on it.
Why even comment on what a "stock" transmission will handle when they are now 15+years old?
Far as my tranny it has been rebuild with a lot less "upgrades" than you would think, mostly stock or stockish stuff with careful attention to clearances and responsible shift kit setup and it is over 7.5 years old and was built back when I envisioned a LOT less power for the car.