Actually the way ammo is modelled in AH limits the effectiveness of the German cannons. AH ammo is a composite of whatever rounds were in an ammo belt IRL. I'm going off some foggy memory here, but the Hispano round had significantly higher MV than the MG151, and the standard "HE" rounds for both had roughly the same explosive content.
IIRC a typical belt for the MG151 was 40% AP, 40% HE, and 20% Mine rounds (thin walled, with a lot of HE content). The AP round was significantly worse than the Hispano, the HE round was slightly worse, and the Mine round was better.
I'm completely making these numbers up, but I imagine it goes something like this for the MG151 (if one uses the Hispano as a reference).
Again IIRC, the AP and HE rounds for the Hispano were about equally effective, so the AH Hispano round is modelled as being half HE and half AP, which were the same anyway, so it gets rated a 1.
The AP MG151 round was about half as good as the Hispano, so that is rated a .5. The HE round was about 80% as good, so it would be a .8. The Mine round was about even, so thats a 1. But, the AH round is a composite round, so the AH MG151 is .5(.4) + .8(.4) + 1(.1). That is .2 + .32 + .1, or .62. So an individual round is about 60% as effective as an individual AH Hispano round. I think I'd rate the MG151's hitting power at between 50-60% of the Hizooka in AH, so it bears out in my head, at least.
The firepower of the MG151 has never been changed, to my knowledge. It is adaquate, but the 2x Hizooka 2x .50 combination of the Spitfire is about as heavy as the 4xMG151 2xMG131 on the A-8, in my opinion. I've killed and been killed many times by 1 Hizooka round to the tail, I've never killed or been killed by a single 20mm round of any other kind.