Can't really answer the question, because some of the blower discharge is expanded into volume at low pressure (only slightly higher than normal low vacuum in the manifold), but some may still be under higher pressure between boost pressure and zero, depending on manifold volume and boost level. A 3-71 producing 5 psi into a 500" RB with a tunnel ram manifold will get the entire flow as CFM at zero to low vacuum, a 14-71 producing 30 psi into a 300" LA with a simple base plate to the manifold will have over 20 psi.
Flow benches don't test density.

HP will not increase linear to boost regardless of blower efficiency, because there is heating and expansion just due to the temperature rise in the engine.
The Wallace calculator simply multiplies existing power by (14.696 + your boost input) ÷ 14.696, and it's wayyy too high (yes, the same mistake appears elsewhere, even on blower manufacturer sites - and still wrong).

Read my article: http://victorylibrary.com/mopar/blower-c.htm