I feel like someone functions better with their regular, stable dose of pharmaceutical-grade opioids in them -- I don't have any medical or moral issue with that at all. It's honestly...a lot like the hair dryer anecdote in
http://slatestarcodex.com/2014/11/21/the-categories-were-mad....
As for the LD50 issue -- yes, opioid-naive (non-tolerant) users will have a low LD50 but they also won't need much a dose to get the effects. As people get opioid tolerant, the dose that will kill them also increases in step. It's not a case of "user needs higher and higher dose but needs to not cross a fixed LD50 threshold". Because opioids mostly just affect opioid receptors (unlike stuff like alcohol, which is hepatotoxic), tolerance means the LD50 increases in step with the effective dose.
The popular picture of the heroin addict who has to use higher and higher doses to get the desired "high" is a creation of prohibition -- there's no inherent need to do anything of the sort if one has proper medical access to the right dose of prescription opioids. Moderate and safe opioid use is difficult under prohibition if only because it's hard to moderate oneself without precise and pure measured doses.
There's plenty of people who engage in moderate long-term opioid use, between pain patients or "high functioning" heroin users. Those people tend not be the face of opioid use precisely because they have access to the drugs in ways that are stable, safe, not life-ruiningly expensive, and doesn't place them in regular contact with criminal organizations. They aren't exposed to the worst harms of prohibition and that's why they're able to keep using (without anyone knowing that they use) and have a stable career/family.
To be clear, none of this is to say that opioids are harmless or that should be handed out like candy -- they're serious drugs and should be used responsibly by people who are aware of the risks/contraindications.