Ooookay, I was tired of all the guessing with damage caps and did some testing.
Result first for the lazy: there is no damage cap for judgement in single player below 2.3 millions (that's where I stopped testing).
Setup:
- LoD 1.11b from 12/2006 (long live warden's invisible update feature icon_rolleyes.gif)
- Median 1.54 with the following modifications.
- Kill Rats in Hatred with exactly 30k life, level 1 (monstats.txt cells EZ849 and FA849).
- Level modifier: 1000% life in Hatred (monlvl.txt cell T3).
- /players8 resulting in 450% life.
-> Total life of a Kill Rat in Hatred: 30000*10*4.5 = 1.35 millions
- Judgement base damage raised from 11-12 to 300-300 (skills.txt cells HM448 till HX448), and to 600-600 for a final test.
-> Total Judgement damage: 1037656 (300 base), and approx. 2.3 millions with 600 base and Arcane Strike active.
The life bar of the monster after one shot was at approx. 1/4, if there was a damage cap below one million, it would have done less damage. And the final test with 2.3M damage killed it in one hit, too.
I raised the monster life and damage in turns, to make sure I didn't hit a life cap in the process.
If there's a systematic error in my test, feel free to point it out. Else rest assured that there is no damage cap for physical spell damage (that's what Judgement is, for all the non-palas out there).
Edit: While I was at it, I tested the skill I'm actually more interested in... Mind Flay. You can't simply crank up skills.txt to a few hundred there, there's a calculation limit somewhere (results in negative display, and no damage). So I just raised the base to a constant 30, which displays 342578 lightning damage. On the char were -43% enemy lightning resistance, which should result in approx. 480k damage. And the Kill Rats sure died in 3 hits.