I'm pretty sure these won't stack because they come from the same ability mod, but I've been working on this character concept long enough that I'm starting to doubt myself.
Benefit: While raging, the barbarian inflicts damage equal to her Strength bonus whenever she succeeds at an overrun combat maneuver.
At 1st level, a siegebreaker can attempt bull rush or overrun combat maneuvers without provoking attacks of opportunity. When he performs either combat maneuver, he deals an amount of bludgeoning damage equal to his Strength bonus (minimum 1). If he has Improved Bull Rush or Improved Overrun, the damage dealt by the appropriate maneuver increases by 2 and he adds any enhancement bonus from his armor or shield (though such enhancement bonuses do not stack, if both armor and shield are magic).
This ability replaces the feat gained at 1st level.
So these will not stack on a successful overrun attempt correct?
oh nice, they do stack! and i also hadn't seen Merciless Rush somehow, ty
you can also do this, if you can't meet the evil worship requirement of Merciless Rush:
breaker rush STR dmg
+
knockback STR dmg
+
breaker momentum
+
overbearing advance STR dmg
However I'm starting to wonder if this is a trap: 15 dmg isn't a whole lot for say, a raging 20 STR total fighter barbarian that's level 7 or 8... you get some minor battlefield control as part of this, but I'm wondering if it's not better to just keep breaker rush as is and hedge your bets with weapon focus and weapon specialization to pile on some real damage. I can certainly see the addition of Improved Bull Rush or Improved Overrun as perhaps going too far down the rabbit hole with this archetype, as you can end up being a 10th level character with nothing to do other than knock medium creatures to the ground with minor damage (anything with DR will laugh at you as well).
I'd argue that the ability to "add any enhancement bonus from his armor or shield" that you get from the improved feats would make the attack magical for the purposes of DR, but its tenuous at best and you still aren't getting much damage out of it.
There are definitely better ways to deal damage, but in a low level campaign where you're fighting mostly humanoids the ability to start dealing damage and controlling the battlefield at the same time without any "amp up" rounds is pretty impressive.