OK got my replacement NEC2500 today. You probably won’t be interested that much but for my own conscience here is a scan of those problematic FUJIFILM03 that I was having trouble with the 6x portion of the burn.
Using stock 1.06 firmware.
As you can see. Much better results. I still get a bit of a dip in speed during the 6x write phase but nowhere near as bad as before. At least they seem to read back OK!
Will back up my firmware and try B5 with some of those Ridisc G04’s and Prodisc R02’s later.