Quite the opposite actually, the mg151 was originally a 15mm gun, it was latter upgraded to 20mm.
Its actually the reason I make this proposition, as the 2 guns system are basically the same but different bore. IRL it should have been trivial to swap between the 2.
But in the context of war thunder were everyone can pull 12G and were most of your fight are against fighter, the ballistics of the 20mm is not it.
Even against a still target, getting any shot at 800m+ is near impossible, in headon the round is so slow that your enemy have the time to:
Pull into the headon, fire a burst and pull out before anything reach them.
I’m sure than if the RLM had to fight in WT condition the 15mm variant would have been a lot more common, but as they focused on taking down big bomber formation they needed the higher damage output of the 20mm.
And despite how hard getting a burst to connect can be, you have horrible belt choice:
Air target has 2 mediocre round and 4 of the good HEI, so you gamble that the correct round will hit.
Armored has 3 APHE, which are decent if you hit something that trigger the fuze, but as you guess, 1 IT round that doesn’t do anything.
Tracer have FI-T which are just worse than HEI, and give all the warning in the world for everyone to dodge, and of course 2 IT round.
Stealth is probably the best, 3 HEI, 1 APHE but a bad AP-I. But you dont have anything to help correct your aim, so tough luck.
Oh, and a bit more: 151 are the worse when it comes to ground target, other 20mm have no problem dealing with pillboxes, but nope, even with the armored belt you need to precisely hit the window to do damage.
Of course, the extreme inconsistency of the 151.
examples, those image come from the server replay, so no de sync business:
p51h that I hit 9 times (only 20mm)
Spitfire that I hit with a burst to the wing, you can clearly see the 3 mine shell (HEI) and 1 IT

I don’t want to derail the topic as this is discussed in many other forum post so I’ll stop here.