artillery support

In the arsenal you can see the efficiency of support weapons. You can see those values if you select the entry under base buildings for your faction.
A support weapon level 52 does the same percentage of damage against a level 58 attacker as if it hits a level 54 attacker.

I have done some more investigation. There is no bottom cap against stronger attackers, only agaisnt weaker incoming units, then the support damage is limited to what it would be as if the unit was the same level as the support.
Regarding stronger attackers, the value is calculated like this:

BaselinePercentage * Min(LvlSupport, LvlOffEntity) / LvlOffEntity

The Min() implies that the smaller of both values is inserted here in case of an attack and then divided by the level of the attacking entity, your previous assumption would be correct.
The value of your Nod Air-Support would drop to 8.06% in that case.