Velocity is transformed across the turbine blade as in a wing that produces lift to px. Velocity is increased in the turbo scoll (divergent duct) from the exhaust. Px in the exhaust pipe from valve to the scoll is not relevant as it velocity that is required and not px.
So explain the science behind waste gate operation, does it bleed off velocity or pressure from the system?
This quote from a turbo-charger manufacturer;
The turbocharger turbine, which consists of a turbine wheel and a turbine housing, converts the engine exhaust gas into mechanical energy to drive the compressor. The gas, which is restricted by the turbine's flow cross-sectional area, results in a pressure and temperature drop between the inlet and outlet. This pressure drop is converted by the turbine into kinetic energy to drive the turbine wheel.