The acceleration of air flow into a duct is inherently a highly efficient process and the difference between even the crudest radius inlet, and the most aerodynamic shape possible is slight, amounting to no more than a few percent. The flow coefficient of a perfect entry would be 1.0 while the coefficient for a sharp edged entry would be 0.6 and a re-entrant plain pipe 0.5.
The efficiency of air flow into a duct is highly dependent on the shape of the inlet, with even minor differences in design impacting the flow coefficient. A perfect entry would have a flow coefficient of 1.0, while a sharp-edged entry has a coefficient of 0.6, and a re-entrant plain pipe has a coefficient of 0.5.