Thanks for the suggestion - personally I feel that the system is already a bit over complicated for many people to want to use it (or understand it), so I would avoid any solution that adds complication that could be avoided. To accommodate your point about users typically not getting high rewards right away, my suggestion is to just extend the bonus time period to 3 months. That way, if I bring in a really high earning user, then I also get greatly rewarded, plus if I bring in low earning users, I will at least get something once they have settled in.