1 damage is OK for a fairly advantage. I think it should depend on the military tech advantage. If you have a huge advantage in military tech, even a non damage attack would be reasonable (eg Mech Inf vs. Spearmen). As research is linked to population, and therefore the overall performance of your whole empire, I find it adequate to represent it in a military advantage. For example: if you are two ages ahead, it may just end in one damage point for your unit. However, if you are three ages ahead in military techniques (exact number is debatable) you should be able to crush your enemies like a fly. This counts especially for air units vs melee units (fe fighters vs [long]swordwsmen, regardless of its era). A straight tech advantage should outweigh production bonusses of the AI.