I think this comment is a very valid concern: Proposal to restrict rewards qualification to a threshold - #51 by Manu
I think right now people pretty unanimously vote no on spam proposals, because they’ll get the increased rewards anyway, and additionally the 10 ICP of the proposer will be burned if they reject. With your proposed change, the voters are now actually incentivized to vote yes on spam proposals, because if it reaches at least 3% yes, they will get boosted rewards. So I expect that many will vote with their own financial gains in mind, and that therefore all spam proposals will reach 3% yes, and the change you proposed will not have any effect.