Imho only people that are outside top20 should be able to plunder and people in top20 should get coupons from supporting other people. This would make things fairer.

Lets say that if you are the strongest in the server by far or if all the comparable power people are in your group then or you shouldn't be able to plunder at all or the support for people you plunder should actually become somehow more useful than no or if you plunder and you win then if the target was supported the target should keep all his coupons because he had no chances from the start regardless the fact he asked or not for support.