I'd love to see your variation tested too but the data-driven "above average" feels much less accusatory and supportive than subjective labeling of "poor student" and "valued contributor". Makes me wonder if poor students are less valued?

Also the fine-grained payment distribution scrollbar controls make me feel empowered and better about my participation in this cause. Getting rid of them would disappoint me.

This reminds me of someone's donation button on some open source project, it was something like:

    Donate [$5 — buy me a beer] [$20 — buy me dinner] [$100 — buy me a gadget]