hi Sean,
You can wind up the boost maybe upto 2,5bar without damaging the turbine, but a Garrett T28 with it's characteristic compressor housing and wheel architecture isn't build for very high boost levels, since it will not be efficient in high boost regions. This will heat up the compressed air very much and also will give very unhealthy back-pressure and other volumetric inefficiencies to the engine. Reading the compressor map is very important with selecting a turbine and thinking about boost levels. Ball bearings shafts don't make turbo's change their characteristics, they will spool up a bit faster and will withstand high boost levels better.
When upping for bigger power always choose a turbine of which the turbine wheel (+turbine housing) and compressor wheel (+compressor houding) are in sufficient balance with eachother.
rgds
habib
