Flat rate spoils are great at the beginning of the game and for a good portion of the game but after the game has lasted for a long time, things get stale and a player kill becomes less and less viable given the lack of return on investment relative to the cost. Escalating Spoils really help incentivize risk taking. It helps keep games short but it is actually too short (particularly in games with lots of players. Spoils are op too quickly. I think it should be a slower escalation, let's say 1+ per turn not per player cash in. As it is, it's +3 PER CASH IN. That's too much imo.
This escalating spoils change will provide more strategic planning for larger games with more players. It slows down the game enough for bonuses to matter but also disincentivizes late game stagnation.