Hi Jab,
>Ah, but the D-9 doesn't have 4 x 20mm
Good observation! :-)
However, the D-9 had the same wing as the A-8 (or A-9) and could be fitted with outboard MG151/20 (or MK108) as well.
(I think this was rarely done, though. It's my impression that the existing photographs of 4-cannon Doras mostly show prototypes or the rare D-11/D-13 aircraft.)
In any case, the Vogtsberger (or "Voigtsberger"?) table lists the impact the addition of the outboard cannon. The D-9 with outboard cannon lost 10 km/h of its top speed and 0.45 m/s of its climb at at Full Throttle Height, while service ceiling dropped by 150 m.
Even figuring in these losses, the D-9 is still superior to the A-9 in every aspect of performance listed in that table :-)
(I actually checked this before commenting on the superiority of the D-9, but deleted the detailed comparison when I noticed it doubled the take-off weight of my post ;-)
Regards,
Henning (HoHun)