OK, I have now burnt through both of the spindles I bought - and the result wasnt too good (burner: Liteon iHBS112 2).
General disclaimer: On Debian there isnt a free way to do bluray disc scanning yet, so I am falling back to basic reading of sectors.
The first spindle seemed to burn OK, but this included a number of bodged burns as I worked on getting software that was actually capable of burning bluray discs properly. On the second spindle, I came across a number of discs where the final sectors were burnt to, but unreadable - I countered this with reducing my project sizes by 300MB so I will never write to the ends of the discs. Of course this is unacceptable for Taiyo Yuden discs, so I won’t be buying these again.
I appreciate the fact that SVP got them in specially after I enquired - when I reported the failures back to them they were intially interested (non-TY unrecognised JVC media code meaning my drive would only burn at 2x etc etc), after 4 days of communication I was offered extra discs due to the bad sectors. ~ 1 month after I said I was interested, I check the ticket to find it had been closed… a further message later to enquire, and I find it has been closed again, which is extremely rude!
So, I won’t be getting more bluray media from SVP… it wasnt mentioned at the end, but it could be that they wanted my media back - which I couldnt do at that point as most were burnt and archived in an index of a few thousand discs (aside from the fact the data is private) - however the free discs werent mentioned on condition of returning the others (I did have two unburnt so I offered to return one, but that wasnt taken up).
Now I need to hopefully source some actual TY bluray media (going by the media code) from a different provider