I'm now convinced it is an issue with Nero 8.
My BDR-S06XLB did exactly the same thing with another BD-R, and with these CMC BA5 it only offers 1x speed. With Ashampoo Burning Studio it offered the full range of speeds and successfully completed a disc at 4x.
The disc above was written by the BDR-TD03 with CDBurnerXP in 32 minutes, which seems correct for 4x CAV.
This BDR-TD03 continues to impress me with its writing quality on all types of disc. On these CMC BA5 it has been consistently better than others (although the sample size is still very small). Here is another disc from this pack written with the BDR-S06XLB.