I wonder if there are any pictures on the damage caused by a MG151/20 AP/I round?
I bet that would be devastating.
The problems with the Bk 37mm and Mk103 is that Tungsten was in very short supply.
For the Bk 37mm though, it could use the APCR already built for the obsoleto 'doorknocker' 37mm Pak35/36 gun, so no loss there.
Using a bigger gun was necessary using APCBC or HEAT.
The 75mm doesn't sound so bad now? but perhaps the Me262's BK 50mm firing Pak38 50mm-type ammo would have been better? (old APCR stock would likely be available here too)