Thought that norbupe had too much affinity for the BBB to be of much use as a metabolite? I know the BBB affinity isn't in itself a no-go, as I've been addicted to loperamide for three years. But if what you say is the case, I'd love to taper. Problem is, I don't think I have the constitutional fortitude to allow myself that self-control.
I too was under the impression that it wasn't that clinically significant & was going from anecdotal experience. I just realized I was thinking of & am now referring to norbuprenorphine-3-glucuronide as opposed to norbuprenorphine itself BTW...
As I said in my experience for me personally I get more opioid type effects as well as pain relief at doses of 2mg or less. I've dosed from 100mcg to 32mg over the years. Back when buprenorphine was more available in tablet form I would weigh out microgram doses uses a scale to weigh the tablet & break down the tablet accordingly in weight/mcg ratio but I digress.
Since using methadone & fentalogues for a good period time & the advent of the strips with having access only to the 8mg strips I find it hard to cut doses lower than 500mcg. 250mcg is possible with the help of the wife without papers but I tend to shake so it's a pain to cut them that small.
I'd gone ahead & typed up like a page worth of links & information before realizing how much of it was contradictory or dated.

As such I've decided against linking them as IIRC you have frequented ADD & are familiar with pharmacology? If so you could read them yourself if interested. As such I decided against making my very long posts even longer. I tend to be very verbose to put it mildly.

The studies I did find appear to be inconclusive or contradictory (at least in terms of what I can access without running into a paywall) but some do seem to imply that a small amount does bind. Considering the potency it may be enough to have action.
Either way from my experience trying doses in the range I've mentioned at increments as low as 50mcg in the long past or 250mcg in the more recent past that under 2mg seems to be the sweet spot for pain relief & opioid "effects". I know that until I got to 2mg or less I didn't notice it that much.
Once I reached the 2mg per day threshold (ideally taken as 1mg or less at a time) I feel my dose. I get some pain relief!

Also on onset of my first dose of the day I get that stomach tightening nausea (pleasant for me) that I associate with a full agonist kicking in. As such based on personal experience I am inclined to believe there is some sort of mechanism in play.
Though said pain relief is less than what it could be if I was properly treated getting any pain relief is a plus. I didn't think buprenorphine was going to help at all until I ran into an old notebook I had from when I first used buprenorphine (as I hadn't used it in years) that mentioned my dosing & that less is more. I ramble though so on to the next part of your response.
I know that I have a lot of self control though compared to average from what I've been told. For example I used opioids IV daily & had no issue putting the needle down & switching back to sniffing when the fent BS became prevalent. Or just in general. I would only IV substances where it was "worthwhile". For example hydromorphone I would definitely IV; where as say oxycodone I would not. I take BA, the rush, safety, etc. into account when deciding to IV or not. In a year where I was actively using opioids but had little access to anything besides "dope" (random shit with some fentalogue sprinkled on top) I IV'd maybe 2-4 times. Basically on special occasions I saved some other opioids for. That or I will IV as needed for pain relief. To me it's just another route no different from any other if done properly (microns, etc.).
Again I digress though.

My apologies. I personally didn't find buprenorphine to involve self-control as I have no craving for it. As I stated I get 100% more effects from sub 2mg dosing as opposed to 0 effect from higher doses. Other than the relief of withdrawal symptoms of course. However; I'm looking for more than relief of withdrawal symptoms as I'm a CPP. Thus I have no urge to dose higher. As dosing higher just makes for less if any good effects which obviously one would not crave. I still have the urge to use other opioids & would love to switch back to something more effective for pain but it's definitely better than nothing.