Another metric to possibly consider, fof/best-answer. How many answers has a user provided that have been marked best answer, more answers = more rep!
Though I don't think this extension is currently tracking totals per user. I still wonder if that should be the responsibility of each extension, or as you alluded to perhaps a data gathering extension that supports many extensions (fof based ones to maintain compatibility over versions). It seems to be a mixed bag of extensions that support this type of tracking stuff. Anywho, just some thoughts!