I was curious and did an enchantment experiment to see how much additional damage they grant to weapons. I have a spreadsheet that I can't attach to the GOG post unfortunately, but I will be able to send it to anyone who asks. I might upload it to a google drive or something too.

Introduction:
This spreadsheet is a semi-scientific analysis of damage output of a weapon in Ultima Underworld 2.
See the other spreadsheet tabs for the raw data.

Methodology:
Character stats: Mage, 15th level, 18 Str, 18 Dex, 24 Int

Character skills: Attack 0, Defense 30, Sword 30

Enemy: Despoiler, Vitality 125, Armor 0, summoned using Summon Daemon scroll (game saved after summoning)

Weapon: Excellent shortsword, hacked in the game files to use 0 damage in the first test and 10 damage in the second test for every attack type. The weapon was also hacked to reach full power charge immediately.

Enchantments: In each test, a “no enchantment” shortsword was used, as well as a separate shortsword that was initially enchanted to “Minor Damage”, then increased using the Enchant Item spell (VOY) for subsequent tests

The “Study Monster” spell (WM) was used to identify the enemy’s vitality before and after each hit

In the 0 damage test, a tally was kept of the number of hits which caused a specific amount of damage. This continued until the enemy was dead. The final hit was not recorded.

In the 10 damage test, I recorded the damage of each hit, up to a total of 50 hits. The game was restored each time the enemy was close to death.

I was surprised at the results of the 10 damage test results for the “Unsurpassed Damage” enchantment, so repeated it a second time. The second set of results was worse than the first!

Additional notes:
The cluebook hints that a character with 18 strength receives approximately a +1 to damage score.
The cluebook hints that a Despoiler has 0 armor, which should result in 0 damage subtracted from any hit.
When attacking, the “Stab” attack was always used.
Attacks that dealt zero damage were not recorded, as it was too difficult to tell whether it was due to missing the attack or from causing zero damage.
From the cluebook, the weapon skills apparently do not add additional damage for that weapon type, they only increase chance-to-hit.
From other experiments, it does seem that the “Attack” skill increases damage, so it was set to zero for this experiment.

Conclusions:
The damage enchantments generally do seem to make a difference. Quantifying the additional damage is difficult
There seems to be a larger bonus for low damage weapons and a smaller bonus for high damage weapons.
What does seem clear from the results is that the “Unsurpassed Damage” enchantment appears to be bugged and does not seem to add extra damage.
It’s possible that luck played a significant part in the results, but the large number of weapon strikes was supposed to average out the effect of chance across the experiment.


Results:
0-damage test:
No enchantment: 100.0% (used as baseline)
Minor Damage: 118.2%
Major Damage: 127.5%
Great Damage: 140.2%
Unsurpassed Damage: 100.8%

10-damage test:
No enchantment: 100.0% (used as baseline)
Minor Damage: 110.7%
Major Damage: 111.2%
Great Damage: 115.0%
Unsurpassed Damage: 112.7% and 101.5%