We don't know very much about the difference between giving from income and giving from assets, from capital, but I think it's reasonable to expect that giving from capital, when there is substantial capital, is a lot less painful than giving from income, meaning from what you're earning. I think there are pools of capital and I think there are individuals with large amounts of capital who can be targeted. In my brief, I mentioned looking specifically at bequests. There is a lot of money there.
The answer I offer to your question is yes. I think having a sliding scale will be psychologically advantageous, because large donation donors are, with very high probability, giving from capital rather than from income.