[Diesel_Dave]

I guess I was assuming that you were not necessariy holding boost pressure constant. Regardless, if the boost remains constant the mass flow will remain constant, however, the exhaust temperature can change significantly. A change in exhaust temperature means a change in exhaust pressure, therefore the pressure ratio accross the turbine changes, therefore where you are on the turbine map changes (even if where you are on the compressor map hasn't changed).