On the subject of penetration, do you have any documentation that puts the MG151/15's relevant air-to-air rounds as having a significant advantage in penetration over the MG151/20, let alone any other 20mm autocannon, or are you just making judgments based on muzzle velocity (compare to, for example, the Hispano 20mm, which fired a much, much heavier round at even higher velocities)?
Also, as far as aircraft armor goes, I don't believe that much of it went any further than having a small several-millimeter-thick steel plate in front of particularly-vulnerable components (such as the pilot) from particularly-vulnerable angles. Old 1940s aircraft engines, especially radials, were fairly rugged though.
Having said that, I have to mirror this statement.
If you look at the first page of this thread, you can see the Armor Penetration Chart for the standard "AtoA" munitions, you'll see that at 300m a square hit penetrates 25mm.
There were alot of redesigns in the Hispano's Armor Piercing rounds. In 1940-1941, the muzzle velocity of their AP rounds was only ~700M/S, much slower than the Mg151/15, and resulted in 25mm of Armor Pen at 200m, 22 at 400m, so we can assume that these were roughly comparable numbers to one another, the Mg151/15 slightly better. (This is the Mk.I AP)
One must note though, that AP munitions in 1941 for Britain only accounted for 1% of the total 20mm munitions produced. (That's some rare toejam.)
The AP MKII, came in 1942, and boosted its muzzle velocity to 750m/s, still slower than the MG151, and boasted 27mm of armor penetration at 200m, 24mm at 400, about par with the Mg151/15. (By now production of AP munitions for Britain increased exponentialy, and was by no means considered rare at this point, in fact, it was probably one of the more common rounds produced.)
In 1942, the Brits realized how ineffective their 20mm cannons were at piercing German Armor (They was getting rocked by them Panzers), and so began the production of the Mk III AP round for use against German Armor (Air to Ground). This boasted the fast 950m/s AP Projectile with 67mm of armor penetration at 200m. While slightly slower than the Hartkernmunition (We are talking AtoG projectiles, so now I am comparing the Mg151/15s AtoG munitions) equivalent, it penetrated more armor. I also must note that this particular ammunition had questionable stability, however the Brits thought the high performance was too good to be sidelined.
In almost every case when you compare the munitions, the Mg151/15 has almost 100m/s more than your Hispano Munitions. They are about on par with one another in terms of firing rate. The Hispano obviously had a heavier projectile and larger projectile capable of containing more HE or I elements in the non AP shells and thus making it much more destructive to the hardly armored Aircraft of WWII, and thus, a better gun. (This is the same reason the Germans abandoned the 15mm projectile and hastily slapped together the 20mm variant of the Mg151)
The fact that a 15mm AP projectile was pretty equivalent to the "Best" 20mm Cannons AP shell for nearly 4 years, makes it a beast (I have no info on the Mk IV AP shells introduced in 44). Let's remember that the 15mm variant was NOT an auto-cannon, it was a Heavy MG, if you wan't to compare it to something in it's class, then you would be better off looking at the Browning M2. Which in my opinion is also beast of a weapon considering it's just a machine gun. Pretty much any gun from the M2s .50 cal and up, is a beast in my eyes.
Maybe I just have low standards.
