They don't specifically mention work output but I find it hard to believe that opium addicts were particularly productive in most jobs. I don't think China was very industrialized at the time so I doubt what we would today conceptualize as a "worker" was a substantial fraction of the population at that time.
I also found this, which may disagree: https://academic.oup.com/book/32900/chapter-abstract/2766221...
The ‘Problem’ of Opium Smoking in Canton Virgil K.Y. Ho
Abstract
Both the causes of opium consumption, and the allegedly calamitous social, political, economic, and cultural impact of opium smoking on Cantonese society were greatly exaggerated in official propaganda and scholarly writings. Opium served Canton and its urban inhabitants many positive social, cultural, economic, and political purposes, making it difficult to over-simplify the phenomenon of opium smoking as nothing but pernicious.
"but I find it hard to believe that opium addicts were particularly productive in most jobs"
well maybe, but do you have any experience with this other than feeling? Same has been said of dope smokers, but many are productive. Also in china a few centuries ago there was no welfare state and for most, no work = no food = beg or starve. So as a non-productive person, you didn't live too long (edit: to be fair, this is just my assumption. I may be wrong).
Abit more, from https://en.wikipedia.org/wiki/Opium#Recreational_use_in_Chin... that suggests something very different:
It is important to note that "recreational use" of opium was part of a civilized and mannered ritual, akin to an East Asian tea ceremony, prior to the extensive prohibitions that came later.[50] In places of gathering, often tea shops, or a person's home servings of opium were offered as a form of greeting and politeness. [...] The image of seedy underground, destitute smokers were often generated by anti-opium narratives and became a more accurate image of opium use following the effects of large scale opium prohibition in the 1880s.[50][51]
Not with opium smokers but I have some familiarity with opioid users and they are generally not good workers. Of course maybe there are tons of opioid users who pass as totally functional people and so I don't recognize them, but I doubt that. Marijuana is also clearly not addictive in the same way as opioids are (perhaps it is slightly addictive and maybe more so now that it has become stronger). It seems to be more like caffeine where its use is ritualized but people don't have a very hard time with cessation if forced to. I would bet daily users of cannabis are significantly less productive than the average population, but I know a couple in high performance jobs, so it's certainly not guaranteed.
I imagine in the China of the early 1900s many of the opium users did die just as many heroin/fentanyl users today do. I have talked to addicts who expect to die of their drug addiction and openly say so.
Edit: I thought about it a bit more and I am aware of some functional addicts addicted to (legally proscribed) pain meds. So I suppose it is possible opium users/addicts were more like this group. I doubt it though because China went to extreme lengths to eradicate opium from the country after the communists came to power.