Max stacks of poison are 3 (dmg capped at 7997, 1999 x 3).

I strongly believe that it's the action that cause poison, not the single istance of damage... or you would be capping it with Hanzo doing his standard combo.

Also yes, Ibuse is a weaker KoH. Since 5.0 removed summon points and summon scrolls from swb, you are getting less points (unless you're buying summon scrolls from sun shop...) each week, which made KoH harder to piece / drop.