I think the most significant and strongest correlations are with the quality of democracy. As I've said, I use a whole series of indicators of democratic quality. On all of those, proportional representations works not only slightly better, but a whole lot better. There is simply no comparison between PR and FPTP in that respect.
What is also important is that it works better for effective government, which has long been the argument against PR. Many advocates of FPTP would say, “Yes, of course, PR is the more democratic option, but we should worry about other things than just the quality of democracy. We have to worry about effective government.” The evidence is now in and it's clear that for effective government, PR also works better. PR systems and consensus democracies have a better record with regard to effective policy-making.