I don't know if you use the ki67 but tailgunning with 20mm's is WAY different than with 50cals. Takes a bit of time to get used to the bullets trajectory, maybe its due to the wind affecting more the larger shells? lots of parameters there...
If I recall correctly, the Betty's 20mm cannon is the same exact one used in the A6M2. You have to aim a tad higher with it (due to horrible ballistics) than you would with the B-29's 20mm, or any other 20mm for that matter. The one thing I've noticed about the B-29's 20mm though, is that it sprays a little, kind of like the Ostwind.
On another note, is it me or does the Ju-87's OBJ seem a tad high (11 OBJ)? Am I missing something here? This is in LWA's I might add...