"Something else to remember is that by going downshot, the static pressure will run higher on some units. At least with an elbow, the air can come straight out the back of the unit and make a gradual downturn through the roof. Downshot, it smacks the back of the unit and has to force its way down. "
Yep, thats a fact. take my Rheem rjnla060 (package unit) for example. The manual shows that you should calculate between 0.05@1400 cfm and 0.1 @2400cfm additional static pressures for downflow applications.
Of course, a really good tin bender can fix that with by fab'ing up a internal elbow with turning vanes. I was able to remove almost 0.15 inwc of static pressure by having an internal elbow w/ turning vanes made and installed.