Found this info from Neuspeed. Neuspeed is a highly respected tuner in the VW/Audi world.
Quote:
The K04 is designed to provide reliable, long-term service as long as its performance parameters are not exceeded. Driving the car at engine speeds above 5800 rpm with 15spi or greater boost significantly increases the exhaust temperature measured inside the K04 turbine housing. This signals that the maximum efficiency of the turbocharger has been exceeded, and that the energy produced by the turbine housing is now converting to heat rather than accelerating the compressor wheel faster. Unless a careful and extended idle-down is performed at the end of every hard driving session, the rapid heat build-up from high boost/high rpm operation causes the turbine shaft to soften, and allows the inconel turbine head to droop. The result is an imbalance that ultimately leads to a shaft failure.