Turbochargers won't overspeed if the outlet blows because that increases the mass flow, slowing the wheel down. Sounds backwards but it is true. What is really hard on a turbocharger is stall, this happens when the compressor outlet is suddenly choked off. The momentum in the turbine/compressor/shaft then tries to generate pressure ratio exceeding the steady state capacity. Flow eventually reverses, over stressing either the turbine, compressor or shaft.
If you choke off the compressor inlet, the turbo will speed up because the compressor forms a partial vacuum on the input, and vacuum has no mass to flow. This should be familiar to anyone that has ever run a vacuum cleaner, and stuffed up the hose.
If you choke down (not off) the outlet, the turbo will slow down because the compressor is doing work by compressing the air.
I have read several times that blowing a hose, or gasket on the output of the compressor will over speed the turbo, but I have never experienced it.
A simple shop vacuum cleaner experiment doesn't show much difference between the outlet blowing free, and being stuffed. The motor clearly slows down when it is stuffed vs blowing free, but all-in-all, you get a much greater difference in speed when the inlet is stuffed, than you get for anything you do to the output.
I will have to look into this some more.
-Chuck