It  would seem "reasonable" that a low pressure area at the exit  will help flow through a duct - no argument on that point.  What the report appeared to say is that the after a certain point opening the exit area wider does not appear to have any additional benefit. (Exit “area” and exit “pressure” are not interchangeable terms) That if the duct is capable of "using up" all of the kinetic energy in your air flow by obstructions, pressure drops  and friction losses then enlarging the exit does not necessarily  add to the flow.

Remember you can not suck air through a duct, you can only blow it through. (Of course you can suck air through a duct – I do it after (and sometimes before) every flight with the fan I have on the back side of the radiator) So in effect if the straw is pinched you can "suck" on it all you want but it won't increase flow {:>).  

If I understood the report,  it appears that enlarging the exit area beyond the frontal area of your core provides little if any additional benefit.  That does not mean cowl flaps never work or provide benefit.  In fact it appears that the better your duct,  the more benefit the cowl flaps appear to have, the worst your duct, the lesser benefit - just the opposite of what you might think.

Ed;

Don’t mean to be picky, but some of these generalities are making me nervousJ.  These things are applicable only when the duct/diffuser is operating at max efficiency – which is rarely the case.

Lot’s of good info.  Thanks.  You’re right; it’s some kind of magic, and you don’t know for sure until you built it and try it.

Al