If I look at this article on the DA42 UAV, the ceiling is 30000 ft, but the engines seem the same.
DA42-Based UAV Tested
Also the Grey Eagle drone has a reported ceiling of 29.000 feet and uses a Centurion engine.
February 2012 Military Power - Diesel Power Magazine
I've been talking to a DA42 pilot and he reports a climb performance at FL170 of about 900 feet a minute, that does not indicate a big loss of power at altitude.