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By and large any converter clutch a car comes with with hold the full stock engine torque. Adding peak hp to an engine doesn't necessarily change that scenario very much because a clutch only cares about torque and the torque your engine makes at peak power even after mods is probably less than what the stock engine made at its torque peak.
Aside from that, there is a big difference between the clutch holding peak torque and engaging during peak torque. This is highly related to where the whole 'don't tow in top gear' thing comes from. Going back to my first statement, in almost all cases every gear in a car will hold that engine's peak torque, including the top gear. Some newer cars may not fit this because the electronic controls can be used to completely disallow you from making full torque without shifting out of that gear, so they could build the trans so that some gears may not hold full torque. But the not towing in overdrive thing predates electronic controls completely anyway. So, using the 4l60 someone mentioned as an example, 3rd and 4th gear actually use the same clutch pack, and it's this clutch pack which typically burns up first in a 4l60. Same clutch pack with the same capacity whether you are in 3rd or 4th gear. The PROBLEM comes from shifting back and forth between gears during high engine torque. If you simply manually control the shifts and reduce throttle during shifts, the accelerated clutch wear that comes from high-torque downshifts while towing pretty much goes away. At that point you can both tow in any gear you want, and also not suffer much effect to the lifespan of the trans.
Minor caveats to above paragraph. On some transmissions there can be a situation where you're at a low enough throttle to stay in X gear, but a high enough throttle to unlock the converter clutch (again assuming you haven't taken manual control of it through mods). In that scenario putting high engine torque across an unlocked converter for long periods like climbing that hill in California someone mentioned, will put a bunch of heat into the fluid. So that's another scenario to watch for and another reason to take manual control of the TCC if you know when it should be on and off.
Long story short, avoid engaging holding elements like clutches and bands under full engine torque if you want a stock or better-than-stock lifespan from the components. OEMs now do this automatically with torque management during shifts by retarding ignition timing or skipping ignition or injection events during full throttle shifts. That's part of why modern sports cars make such distinct noises from the exhaust during shifts.
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