First thing is horsepower doesn't break anything; torque does. The factory torque rating for the TH400 (3L80) is 80 which is just about as high as it gets There is a 90 rating in the 6L90 out of the new supercharged Corvette 427 model, and on Alison transmissions used behind a diesel engine in trucks and buses.
In stock form it will handle a BBC easily (even a potent one that will break a Muncie four speed). People complain in posts that the TH400 has a lot more internal friction (parasitic drag) but this isn't strictly speaking true. When in direct gear it has no more parasitic drag than the TH350 (now called a 3L60) or the 4L60.
Where the you can say it is sucking up power is in shifting.
All automatics bring the internal components to a dead stop and reverse the direction of rotation on up shifting. That is a matter of a lot of energy lost to inertia. The more massive the part being reversed the more energy it takes to stop it and getting it moving again in the opposite direction. This is where the TH350 has an advantage. It is made of weak but light weight aluminum. The TH400 is made of physically bigger parts than those used in the TH350, made out of cast iron that takes a lot more energy to shift.
Knowing how heavy the TH400 is, just think of the work (horsepower) is being done to stop the internals from rotating, and then to bring them back up to speed in the opposite direction upon a shift at 5,200 RPM from first to second in a drag race. It is because the parts are made out of cast iron that it has the strength that it does.
What you are replacing on a high performance rebuild is the drums (shells) that stop the rotation. These HD shells have stronger splines at the top of the shell. The four gear input carrier is replaced with a five or six gear input carrier, stronger springs to increase line pressure (clamping force on clutches and drums), solid aluminum spacers to replace the governor shift piston, and improved friction material on the clutch discs and the drum bands, and a shift kit or reverse pattern valve body. If you really want to throw money at it for a blown BBC application you replace the case, and the gears to match your power band.