No. Mithing up only increases your strength/damage, and plunder is solely based on your build and the target you're hitting.
Shadow, this is a place for kaw questions only. Pleaseuse a search engine for those kinds of questions.
When I attack people they lose like 160 mil but I only gain 70 mil. And most of that is from ally bonus. So what happens to that gold? And is there a way I can tell how much gold they lose?
The amount a player loses will depend on the gold they have out. It doesnt go anywhere, just vanishes. The amount a player loses changes for steals and attacks, but how much they have out will make a difference.