I'm also new to the wonderful world of EDF's... It seems from a preliminary googling that a given engine/fan/battery combo has a maximum thrust and ducting can either aid in achieving maximum efficiency or hinder it.
From what I have read, it would seem that the most efficient ducting would tend to accelerate the air entering the fan (still no idea HOW, but hopefully I'll find that info somewhere).
I am leaning toward the engine/fan/battery combo (some of those motors are up in the "wide open dremel tool" rpm range) as the main limiting factor.
I wonder if it would be better to have multiple rotors/stators in each EDF unit to give some great thrust without going broke on the latest brushless motors and megawatt li-po's?