The way i figured it, probably incorrectly, was by looking at the points values of greater leadership values whilst picking troops for QB's
If you bump up the L value of a squad leader, where he controls 11 men the cost is considerable.
Whereas if you apply the same bonus to a company commander the cost is small
So i feel that the Leadership bonus affects the moral/abilities of units DIRECTLY under that leader, BUT ANY EFFECT GIVEN by the speed of infomation exchange up and down the C2 chain must be small due to the small cost of making your 3 man company commander team +2L
So for my games i try to select +1L for all my squads and dont worry too much about higher up the chain, i just make sure i keep C2 which is about posistioning not about bonuses, and that setup makes for a very tough force indeed.