The muzzle velocity is comparable to German 20mm cannons.
But the bullet drop is practically non-existent… WTF!!!
I’m planning to read through this entire discussion at a later date, but I’ve got to go to bed soon. For now, though, I will say that the reason the Hispanos (if I remember right that’s what the cannon-Stang uses) have a flatter trajectory is that they have less drag, not just that they have a somewhat higher muzzle velocity. That is to say, while they do leave the barrel at a higher speed, the primary advantage is that once doing so they lose that speed more slowly (which is more the more significant of the two advantages).
If I remember correctly, this is primarily due to three things. The first is that the minengeschoss rounds have a great deal of HE filler, and by nature this means their velocity and aerodynamics are more limited than the Hispano rounds. The second is that the MG151/20, Germany’s main 20mm gun, was originally the MG151/15, a 15mm cannon. The gun was refitted to fire 20mm rounds, but is still relatively short and low-velocity for that calibre, as it was originally designed for a smaller round. The third is that in real life ww2 combat you tended to be firing within 100-200 meters, not 700, so the difference in velocity made less of a difference; if you want to see how this plays out, play a bit of Sim or watch some videos of IL-2 Sturmovik fights.
This can be seen on the Bf 109 F-2, which features an MG151/15 in the nose, the Bf 109 F4, which gets them as gunpods, and in one of the Do 335s.