There are 2 reasons why the NON-sti tranny's fail.
1. The gears.
2. The tranny casings themselves.
All decent builders i've talked to say: "ALWAYS go STI," if you are going to put serious power through them. You can get away with aftermarket gears, but from what i understand.. the casing itself is what FLEX's under HUGE load... which breaks the gears. You need the gears to be strong, but you also need the Tranny casing to also hold the power.
Ask in the tranny forum.. they know far more.