[Christ]

Speculation here:

I think at high speed, there would be a choking effect, that would not allow you to realize the full A reduction.

Based on what I know about fluid dynamics, which admittedly isn't as much as several others on this site, the "pipe" as it were, would have to be completely straight. Any taper in the front of the pipe would cause further compression of the flow as it was pushed into the "hole". Any expansion in the pipe after the entrance would allow the flow to expand accordingly, thus slowing it down, causing a back-pressure effect.

The pipe itself would contain it's own boundary layer, which also cuts down on the net A deduction, albeit minutely.