So, that took a surprising amount of digging, but I did manage to find this article from 1981 detailing the affect of various drug combinations. In general, it seems that acetaminophen has a real and sustained increase in the effects of various narcotics. It’s more noticeable in oxycodone (and surprisingly, really really small for codeine) but it’s definitely there for hydrocodone, and basically the effect is so strong that taking one without the other is actually more risky, because you need more of a substance with a bad side effect to get the same pain relief.
Now, the exact formulations have gone down as you said, they use 1000 mg acetaminophen in this study and i think standard of care would really suggest against that now. But even at lower doses the effect is still seen. So I think that it really is risk-benefit. Without acetaminophen you need a lot more of an addictive substance in your system to get the same effect, and thus a greater chance of getting addicted. Or alternately you have to kill your liver to get the same degree of pain relief from tylenol. So while it has it’s dangers, the combination is still worth it.
Also, that first sentence of my second paragraph above should read “This also isn’t to say that your theory isn’t correct”.
Also, another fascinating thing from that study? Excedrine is acetaminopen, aspirin, and caffeine. I always thought that the caffeine was there to counteract the drowsiness effect of the two analgesics, but according to this it actually makes them way more effective. a combination of just acetaminopen and aspirin is actually slightly less effective than the lone drugs, but the two plus caffeine is almost twice as effective, depending on dose. The more you know.