So I went to determine the Mk 103 in game damage by the good old "rounds to kill hangar" method - the only way we have to assess damage done. While we only can fire the MG 151/20 and the MK 103 in unison, the ROF of both weapons are known and so is the MG 151/20's damage value (3.55). According to my findings, the MK-103 has a damage value of about 12.3, which is only a tad over the Mk-108's 11.6. Maybe someone else can do his own tests to verify this (or to find the error in my thinking)
You start firing at longer range in the mk103 (the HO scenario), its ls so lethal in its nose mounted configuration, nothing survives.
Against B26 I normally got one kill and 1 or 2 smoking planes in the A8 with 30mms, against the B17 normally 2 kills and one smoking (2 or 10 o'clock slightly higher).
In the Me410 normally you start shooting at longer range, normally I got all 3 B17's and vs the B26 i got 2 and one smoking.
Note I did not count bullets, often the ME410 was messed up enough so you had to land , whereas the A8 was almost untouched, except when a certain 99000 was flying it

In the A8 with 4 cannons you could expect less unless you shot from 11 or 1 o'clock, then you get all 3, B17s and in return you had a smoking or dead engine.