Derrault, your maths on expected wounds for the shotguns is wrong.
3.5 hits get through cover
1.75 shields are rolled
1 shield is cancelled
2.75 wounds go through.
Of course, just averages don't tell the whole story. I worked up the full cumulative probability distribution, which better models pierce, but the result is pretty much the same.
Your maths indicates you think pierce removes a defence die, rather than cancelling a shield. This misunderstanding is probably why you disagree with everyone else :).
Edited to add: another flaw in your method is by not correctly modelling the rolls you are overestimating the value of cover. Cover only works to it's full potential if at least two hits (not including crits!) are rolled. On white dice with surges, 1/3 of all successes will be crits! This is actually one of the big benefits of the z6 and fleets in general that you are not capturing, which is skewing your perception.
Edited by colki