Here is the test date from 1802 comparing Gribeauval to AnXI guns. Note the degree of elevation. It is noteworthy that the howitzers cant obtain the ranges qouted by Scharnhorst.
100 shots fired at each point of elevation to determine an average range .target measured approx 2m square (2 toise)
The Gribeauval 6.3pouce howizter had a maximum range of 986m at 6 1/2degrees. 3 hits
The Longe Porte at had a range of 1475meters at 6 1/2 degrees. 7 hits
The AnXI recorded no hits at 6 1/2 degrees or 6 degrees. Maximum range was determined at 5 1/2 degrees elevation of 1301m. 7hits
No hits were recorded beyond 7degrees elevation, but that is not to say the shell did not travel that distance. The field guns could fire up to 10 degrees elevation and record a hit on the target.
Beyond 15degrees elevation the howitzer axles did not last the 100 rounds.
Reference: SHAT 4w 648
I wonder what exactly is meant by high angle? French howitzers could not fire 45 degrees, let alone 20degrees. I suspect Scharnhorst extrapolated the data or used mortars.