Not your power, your torque and the way it's applied to the gearset Your build didn't fail, your gearset did. The metalurgy is still factory in any Stage I build.

Talk to Jon, Shep, or any knowledgeable Evo transmission builder about the relative strength capacities of the Evo gearset, and they'll tell you that 4th is the weakest link in the chain. Repetitive stresses will cause failure at levels below original ultimate strength, as the teeth develop more stresses at the roots of the teeth. That's why shotpeening is recommended, as the compressive forces strengthen the surface layer of the stressed parts.