Alright, I did some testing.
I made a .pud with a castle, lumber mill, archer and 1 pig farm.
The pig farm has been very victorious in a couple of battles.
I edited the lumber mill to have 0 armor.
And the archer to do 0 basic and 0 piercing damage.
As expected, the +1 upgrade allows for 1 to 2 damage.
The second +1 upgrade still allows for 1 to 2 damage.
And then the +3 upgrade..... did nothing!
I re-edited the map and gave the archer 3 basic damage.
This time the first +1 upgrade didn't increase.
The second +1 upgrade now allowed for the 5 and 6 damage to pop up against the 0 armor.
And against the castle, the damage went from 0 to 1-2 damage after the first upgrade.
The +3 upgrade didn't increase the damage against the lumber mill. But suddenly the castle started taking 5 and 6 damages altogether as well.
And while bloodlusted rangers are a thing. The basic damage was always nulled, even the double one. It is just that there is no damage increase by the +3 piercing against 0 armored units.
Rangers and Berserker stand on equal grounds...In fact...
Berserkers have healing and now have a slight better chance than a Ranger.
Well..."now" is not the correct word. It was always that way. And I always made an assumption.
Either way, knowing this. I can think of a new table of results, all possible upgrades:
3 basic, 6 piercing
0 armor: 5-10
2 armor: 4-8
4 armor: 3-6
3 basic, 7 piercing (+1)
0 armor: 5-10
2 armor: 4-8
4 armor: 4-8 (+33%)
3 basic, 8 piercing (+2)
0 armor: 6-12 (+20%)
2 armor: 5-10 (+25%)
4 armor: 4-8 (+33%)
9 piercing (3B -> 3P)
0 armor: 5-10
2 armor: 5-10 (+25%)
4 armor: 5-10 (+67%)
10 piercing (3B -> 3P +1)
0 armor: 5-10
2 armor: 5-10 (+25%)
4 armor: 5-10 (+67%)
11 piercing (3B -> 3P +2)
0 armor: 6-12 (+20%)
2 armor: 6-12 (+50%)
4 armor: 6-12 (+100%)
Obviously it is still worth getting the upgrade.
The first +1 upgrade is still, almost useless. But nessesary for the 2nd +1 upgrade.