Originally Posted by Mister Two

Depends on the turbo...



When a turbo is pushed beyond its efficiency range, the amount of air flowing past the compressor wheel becomes less and less for every unit of gas going past the exhaust turbine wheel. The faster a turbo spins, the more heat is passed onto the air as a byproduct of the internal friction from the moving internal components. As the turbo is pushed beyond it efficiency range, it continues to produce more and more heat, but flowing less and less air per unit of exhaust gas.

This excess heat applied repeatedly, causes stress in the form of heat and friction on the internal components, as they are operating outside of thier capacity to lubricate and support the rotating compressor wheel. This reduces the overall life of the turbo and can cause failure in the turbine bearings after prolonged use outside the efficiancy range of the turbo.
For Example: