Why do you say this? ccrbc got to 17,600. And having the 100fps rate of climb he had is not unexpected at or near the service ceiling. Rate of climb slowly reduces as you go higher until you reach the ceiling. It is also dependent on temperature: service ceiling is defined at ISA conditions.
And you did not answer my questions regarding airspeed and mixture, both of which are critical. And that would be INDICATED airspeed, not true or definitely not ground speed.