I would like to throw in my expert knowledge on the Archer, but I've never seriously used the called shot. Why not? I didn't need it [img]smile.gif[/img]
They are already more than good enough without it
But if I'd have to guess, I'd say that it is, indeed, cumulative. I don't know about the stacking bonusses though. It seems like an either/or/or/or/or to me, just like with the chromatic orb spell.