I am replying mostly as a learner in this case, because I don't know a lot about advertising. However: that model will work, in my view, only if you can guarantee really outstanding results.
Also, in my opinion, you need to ensure you get paid appropriately for outstanding performance: in other words, jf you make 5x the original target, then it stands to reason that you should receive a windfall as a result. And your customer - in theory - should be quite happy to oblige for an outstanding result obtained.
But as I said, I am mostly a learner in this case, but I wanted to throw my opinion out there anyway, if you don't speak, your misconceptions don't get cleared.