Better flow and less backpressure minimize the energy requred in the exhaust stroke to push the exhaust out. This means more torque going to the flywheel.
Getting the exhaust out of the cylinder efficiently is just as important as getting the intake into the cylinder efficiently. You can put the biggest turbos on an engine that you like, but if the exhaust is too restrictive it will rob the engine of power during that stroke.
Tuning has a lot more to do with the power output of the engine at peak VE, which is where the engine most efficiently breathes air both in as well as out. As you exceed the engine's VE, maintaining torque more heavily depends on the exhaust side in a turbocharged applicaction as it is typically the biggest source for loss of power with the enormous amounts of exhaust gas flow.
