extreme spread: basically the difference from fastest to slowest FPS in a string of shots.
Over on 300blktalk the theory is that the light crimp helps with neck tension consistency since most people are using converted brass which has all kinds of varying tolerances and thus varying neck tensions or at least the possibility is there vs. single head stamp REAL 300BLK brass which should, again in theory, be more consistent than mixed head stamp converted brass. I dont believe most people are doing it to avoid setback.
For $15 for the Lee FCD, its a cheap price to pay even if its just for testing. If it improves your ammo, its a good investment, if it doesnt do anything, well you are out $15 which isnt much in the grand scheme of this game.
But, with all that said, I have seen some incredible groups from shot strings that had crazy(like 100FPS crazy) extreme spreads. 300BLK seems to have a mind of its own some times and what shouldnt(high ES) produce good results does and things that should(low ES) produce good groups, dont.
Where in IL do you live?