The potency isn't the basis for the schedule. The addictiveness is the quality that determines this, specifically, the abuse potential relative to drugs in other classes.
Buprenorphine belongs in CIII because it is more addictive than diazepam, but less than morphine, cocaine or meth (the big three of CII, I suppose). Whether tapentadol belongs in CII is definitely debatable. It is certainly less addictive than heroin or ketobemidone, but it's hard to say that it's abuse potential is greater than ketamine or paregoric. I'd certainly say that it has greater abuse potential than benzphetamine or marinol, but I'd say that secobarbital or hydrocodone in any preparation are way, way more abuse prone than tapentadol.
However, the law does not require that it be more likely to be abused than substances in CIV, for instance, and while strictly reading the law would make you think that a drug in CII would be inherently more prone to abuse than a drug in CIV, but it doesn't require that it be more abuse prone than every single drug in that category, so in practice there are some substances in CII that have less abuse potential than some drugs in CIV. Moreover, if you consider drugs in CI, the relative abuse potential does not matter, any drug deemed to have abuse and addictive potential but which has no FDA approval is automatically placed into schedule I.
We have a real need to revamp our system to take into account the reality that a substance's abuse and addictive potential (and therefore the legal penalties associated with illegal possession, sale, etc) should be considered separate from the the medical restrictions. For instance, in terms of potential societal harm caused by recreational use of amphetamines, their addiction and family destroying potential, justifies their placement into CII. The current system bans refills on CII drugs, but these are drugs which, for the vast majority of prescriptions, long-term use is not only justified, but necessary for the treatment to work.
Opiates are even more addictive than amphetamines, but in some ways the social ills of their abuse is more limited- where amphetamine addicts tend to do truly insane things during binges, and are not infrequently violent, the harms associated with addiction to opiates tends to be limited to harms experienced by the user himself and some harm to others associated with attempts to acquire more opiates. The majority of prescriptions for opioids are not for conditions that require long term treatment, so the prohibition on refills may make more sense in this case. Still, this should be considered on a product by product basis- vicodin is not a good drug for long term use, and the majority patients needing it won't be unduly harmed by such a prohibition. Duragesic, on the other hand, is absolutely a product where the majority of patients will using long-term, and many of these are patients who are harmed by the prohibition against refills.
There are other benefits to such a system. For one, it would work to place every approved drug into different categories. While a drug may not have addictive effects or recreational uses but still need special controls- such as thalidomide (which is available for prescription in very controlled circumstances).
There are other benefits to a more nuanced scheduling system. You can avoid the dual scheduling of drugs like dronabinol and GHB by placing them into different categories controlling the current aspects. This may be more easily done with GHB than dronabinol, but overall I think the system works better than our current inherently contradictory and deeply flawed after decades of alteration and revision.
I believe the British use a system similar to the one I propose here.
I probably should have posted this elsewhere... hope you find something interesting in it.