It makes sense I've been told that was where my head was for years!http://www.engineeringtoolbox.com/equivalent-diameter-d_205.html
has the formula
equivalent diameter = 1.3*(H*W)0.625
In the grand scheme of 5% or 10% doesn't make much difference as we are talk 8" vs maybe 6" ducts.
The swirling in a duct is bad and introduces extra losses. Air naturally straightens out over distance in a duct.
Round duct is used because it has less surface area, less weight and cost and in big plants it is custom.
For me rectangular lets me choose arbitrary sizes and is much easier to build custom transitions, corners and hoods.
All my tools have rectangular to round transitions for ducts (except my bandsaw - not sure what I am going to do yet), so even that is easier.
The Cincinatti Fan documentation actually does double the length of vertical sections for SP calculations, but the main reason I mention no vertical ducts is that the target 4000 FPM is based on vertical ducts and a more modest 2500-3000 FPM is generally quotes for horizontal ducts (to keep them clear).
For constant volume the SP loss varies with the 5th power of the diameter of the duct so going from reduces the SP loss greater than 4 times.
Time will tell and who knows the whole thing maybe a bust, but it is pretty easy and cheap to build the ducts and try!