understand what you are saying Bronk, if you look at this chart for the 190A-8 you will see that ballistic arc for the inboard 20mm's should be at the sight line at 550m (601 yards).
so (in theory) if you set the convergence to 600 yards then you should get a good group in the gun-sight plane with a .target command at 600. However, in practice the pattern seems to be below the gun sight plane. Not enough of a major error in my opinion, but when ever I make a custom sight with a long convergence I always run tests to see where it hits.
This test with the convergence (and target) set to 600 yards shows a clear pattern of hits below the sight plane.
Now with the convergence (and target again) set to 200 yards this shows better distribution above and below the sight plane.
[NOTE:] both tests were flown on auto-pilot at 1000 feet and 335mph TAS.
Now I did the test with the P-39Q in a similar manner and got different results. The long range shots showed a consistent spread around the convergence point, so it appears that accurate convergence may be dependent on the specific aircraft model.
So the answer to the initial question may be yes, the important factor to remember is, test the sight with the .target command, to ensure the sight functions as expected.