Given the fact that the Stuka is a dive bomber really only suited to attacking ground vehicles it needs a little bit faster reload times.
The time between bomb drops should be shortened about 25% this would allow a stuka to bomb, climb, and bomb a second target.
The reload time after hitting a resupply should be shortened by 50% at least. Unless you have the auto cannons you have to spend a significant amount of time loitering around waiting for the bombs to reload before being able to make a run.
If the plane was more suited to air combat this wouldn't be an issue but it really isnt especially up against a Spitfire.