I like the idea of a tiered system. I think the biggest obstacle to practically implementing specific gains from specific levels would be inflation and dilution. So I'd make a "twist" of the proposal, implement a "rank" / star system where these ranks/stars/tiers are always aiming a percentage of voting power. Say 0.00001%, 0.0001%, 0.001%, 0.01%, 0.1%, 1%, etc. In this way you solve the "moving target" of SPs-inflation-dilution.