As I said before, I've had both transmissions. I had 2 T-5s: one of the "puny" ones in an 83 Camaro V6 and one of the "V8" models in an 83 Z-28. The LG4 (standard performance 305) in the Z-28 was more than the T-5 could stand. The mainshaft bearings were toast and it made noises like cement mixer in neutral.
I've also had a Tremec 3550 in a 67 Camaro with a 375 hp 355 V8 and it never missed a beat. One of the smoothest, best performing transmissions I've ever abused.
T-5s (especially WC) are good trannys...for non-performance applications. The overdrive is great. The one we had in the V6 Camaro never misbehaved once. But it wasn't putting up with any power to speak of.
A fella can do what he wants, but I sure wouldn't want to try it behind a 400+ hp motor.
Btw, I'm not wonna them guys who buys into every "scary story" that I hear. For example, a common story about a TH700R4 tranny is that you'd burn 'em up inside 100 miles if you didn't have a proper method of locking up the Torque Converter Clutch. So I tried it. I put a TH700 (an earlier model, not the 87 or newer "improved" trans) into a different 67 Camaro and ran it for 6 years behind a mild 350 motor without ever connecting the TCC circuit. I did it just to prove a point.
I also ran HEI distributors when everyone said that they ran out of power at 4500 rpm. We found out that as long as they had a full 12v to them, they ran fine...all you had to do is eliminate the resistor wire or ballast resistor.
And the old story about 400 smallblocks always overheating: sure, they'll overheat if you run CamelHump heads on 'em and don't drill steam holes. But I've got 2 406 motors right now that never run over 190.
So I'm not going by "old wives' tales" or "everybody knows" type of reasoning. I'm using my own experience as a basis for suggesting that a T-5 (even a WC) is less well-suited to high performance application than a 3550.
