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This is the way I understand the whole turbo diesel thing,
Exhaust comes out of cylinder at high velocity, hits turbine and spins it, loses some velocity & heat, the turbine spins the compressor wheel which draws air through the intake and compresses it to go to the manifold and cylinders.
If you increase pressure(reduce vacuum) before turbo, it spools up quicker and requires less power to compress intake air, hence less exhaust pressure drop across turbine and exhaust travels out faster.
I imagine this then reduces pumping losses from the engine, the engine is pumping exhaust out if you restrict this with a turbo, it needs to pump harder. So the higher the pressure is before turbo, i.e. low vacuum, the less power drain there is from the turbo.
This can also be improved with a larger exhaust, but there is a lot more science in designing an appropriately sized exhaust.
Polishing manifolds and improving flow through the engine is really no different to the aerodynamics you do on the body, I think of it as engine aerodynamics, if the air flows better through the engine, then this reduces pumping losses, hence reducing overall fuel consumption.
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