A stack of 12 units has the same stack bonus as a stack of 8 units.
if there is no detriment to having over 8, then it seems to me like the best option in a battle within a single hex would still be a mass stack, for example
if you had say 4 stacks of 8 either all are being granted the same bonuses (leadership etc.) say 8 or there is one stack which has a greater bonus than the rest (still 8)
in the case where all stacks receive the same bonus of eight then being in a separate group or all together has no real difference, as all units receive the bonus of 8
in the other case where one group is greater than the rest, each group will receive a bonus of 8 - some x except for the optimum group which receives 8
if you brought all units into a single stack they would each receive 8 as you have explained above
so then why would erfworld consist of a bunch of stacks of eight within hexs, when it would seem best to have all units within a single stack (granted i would say that stacks of 8 would make sense in scouting parties off alone in adjacent hexs but thats it)
Now, (and I think this is part of your confusion, apologies if I'm clarifying something you know), the other important thing is that while different kinds of bonuses stack, bonuses of the same type dont' stack ON EACH OTHER. So you can have a caster bonus and a dance fight bonus and a leadership bonus and a stack bonus going on all in one stack at once, but you can't get TWO leadership bonuses because you have two warlord in the stack.
I didn't know this, i would like to know where its written, this seems kinda weird to me if it were true what would be the benefit of a chief warlord granting part of his leadership bonus to other stacks, because unless the warlord has a massive bonus the reduced bonus being granted would be overridden by another warlord in the stack?