Well any model GTST that has steel wheels (think r32 might) can run higher boost, the other GTST turbos (33/34) have ceramic turbos which means that it will crack under to much boost... not sure what wheels the t28s have and not sure whats safe on them, generally dont have to many kms on these units so can run higher boost through them anyway
Its not to do with the b/b or bush bearing nature of the turbo, its to do with the compressor/exhaust wheels on them
Someone feel free to correct me if im wrong?