Leonard's third year as compared with Kanter's. PER's are similar (EK 15.6, ML 14.8), but Kanter's usage rate was much higher, ant Leonard's WS, WS/48, BPM, and VORP are all vastly superior. With the likely assumption that Leonard will be getting consistent minutes all year, it's a decent bet that he will be able to put up counting stats that will induce offers upwards of $15M/year.
However, that being the case, if I'm Leonard's agent, why would I let him sign an extension? He's likely much better off gambling on himself.