Put a TKO beside ANY T5 and you'll see that the TKO has a bigger, stronger transmission case. The primary issue with all T5 versions is that under extreme loads, the case starts to spread. This shifts the load on the gears from the center of the gear wear pattern to the tip area. Then the gear teeth start to overload, wear excessively, and ultimately fail
Tremec TKO - notice the width of the gears.
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T5 - notice the gear widths, be aware that is a much closer picture that the one above, so it takes some thought to get the right perspective.
Diagrams courtesy of Tmoss & Stang&2birds
All other things being equal, the wider gears will transmit more power with fewer problems. Add the additional case strength, and the TKO a better choice. The TKO is roughly 25 pounds heavier than the T5, and that 25 pounds is the extra metal in the gears and transmission case.
For those with concerns about the shifting, I have a Tremec 3550, the precursor to the TKO and it shifts very well, smooth and fast. Use the proper lubricant and break it in correctly, and it is a great choice for a high performance transmission.