Not sure if anyone has noticed this - I only noticed because on a previous run I used weak poison (my own alchemy skill of 2), and yet the fight seemed easier than my current run, where I've not only got a more combat-focused character, but I also used better poison.

Checking the scripts, I found the variables for "low" and "high" poison are set in reverse (the below from Teron_Mine_01.xml - the possibly erroneous lines are in bold):

...
<dlgPart>
<portrait />
<speaker_name />
<npc_text>The guard throws you a few coins, tells you to get out, and just like that, your business is concluded.</npc_text>
<onLoadScripts>
<string>if(aod.quest_mineWine_spikedHigh) aod.quest_teronMine_lowPoison = true;</string>
<string>if(aod.quest_mineWine_spikedMedium) aod.quest_teronMine_mediumPoison = true;</string>
<string>if(aod.quest_mineWine_spikedLow) aod.quest_teronMine_highPoison = true;</string>
<string>aod.quest_teronMine_poisoned = true;</string>
</onLoadScripts>
...

Which explains why simple rat poison has the guards vomiting, while the hard-core poison has them only sweating...