What Lalley and I (http://papers.ssrn.com/sol3/papers.cfm?abstract_id=2003531) prove is that QV is provably the only system of vote pricing that gives you efficiency always, so in some sense it is the best possible improvement, at least the best one that is robust. In more narrow circumstances in finite populations you can do better probably, but it requires a fair bit of detailed knowledge of the situation and a lot of complexity.
One question that comes to mind is whether quadratic voting is the best improvement. I look forward to giving this a read.