Honestly speaking, I'm not really sure any more whether it's norbuprenorphine that is more responsible for central mu opioid effects of buprenorphine. It's a P-glycoprotein substrate so its CNS penetration is strongly inhibited, it's actually largely responsible for peripheral side effects like slowed respiration which I definitely can often feel (it's quite pronounced especially if buprenorphine is combined with another drug that does that). Norbuprenorphine itself is also very weak as an analgesic compared to buprenorphine which is not necessarily a sign of low CNS concentration, but in my opinion it's doubtful that norbuprenorphine would have a low efficacy at mu receptors. Also, in PGP-deficient mice the analgesic effect of norbuprenorphine was more pronounced which suggests that PGP negatively affects CNS penetration (
source).