Yes there is a penalty on plunder once you exceed the max ally bonus. It's the dev way of balancing the game; the more allies u hv, the higher attack success probability you get at the cost of lower reward. It becomes significant after you accumulate trillions of allies (which is not a lot these days).
the higher ur stats is the less your bonus plunder at the end of the eb coz you are a bigger guy to fight the same stat enemy
As i converted to t5 l1's my plunder on haunts went up about 1.5m per hit. Since i started building t5l2-3 my haunt plunder has gone downward about 1m.
Higher stats than the "sweet spot" for an EB will reduce plunder, stats are affected by equipment, BFA, buildings, reset items, and even pro packs. That needs to be kept in mind also
Your strength is increasing against an opponent that is staying the same size, therefore effectively weaker. Your plunder will drop. Your ally plunder will remain the same