Is easy to test, buy ally, hit EB, buy another hit Eb, see if payment increased. If not you are at MP
If you go into the 'Guides' section of forums there's a 'what is Max plunder?' Thread by Kizzmo that will answer your question in more detail
Buy a ally and hit the a EB then note the "Plunder From Allies" then buy one more ally n hit EB again n check if the Plunder From Allies had increased or not if it didn't increase then that means u r at ur max plunder but if it increased then just repeat the above procedure.