If you're on a stock open diff (from a P5, read: haha) or the 'newer' MSP diff (not the recalled unit)...gears and box will hold fine with a better clutch, up to 275-300whp if you're not too hard on it...thats more than the bottom end will without VERY careful tuning...
The bottom end is what you'll want to pay the most attention to...There is no magic power number...or magic boost number...that is a give away for a grenade...but a combination of the two, and mostly dependent on the tuning. Despite what you'll read, the stock connecting rods are short and relatively strong...the pistons themselves have nice deep compression rings too...so the engine is relatively good at handling a little detonation...but its heat wash, imo, that is the killer. The stock assembly uses press fit wrist pins for the connecting rods. That in itself is common, and not a huge deal...but the pistons seem to not hold up to prolonged heat very well...the oil rings and drains are easily clogged without frequent oil changes, reducing piston cooling, and some outer pin lubrication...the pistons will swell considerably under higher temps, clamp on the wrist pin...and with pressed pins, that just removed any articulation between the small end and the piston...and you'll do any number of bad things to the engine...bend a rod, break/spin/eat a big end rod bearing...destroy the crank bearings (if the rod holds)/cradle...etc...
its a temperature problem...not a power problem...but those are directly related...so things to not skimp on if you are using the stock bottom end is oil cooling and tuning...running a little richer will hurt peak power, but will keep things a little cooler...meth injection can be used for the same thing. But when done right, the assembly will hold fine for 250 to 275whp with a complete stand alone properly tuned and guarantees on accurate oil temp readings..as well as water temp...before you'll need to worry about the gear box...but its safer, easier, and somewhat cheaper to just rebuild the bottom end with floating wrist pins, fully forged pistons and con rods...than babysitting the tune every second when driving hard...the built assembly's will put up with WAY more temp problems than the stock stuff will...