I thought the RPM max was 6K for these motors?
Anyway, a 15 HP motor will kick a 9.9 butt all day long. A 15hp has soo much more low end torque and throttle response then a 9.9 does. Top end they're only a few miles apart but it takes a bit of time for the 9.9 to max out, the 15 does it in...