Ok, got my Pioneer DVR-216 burner. Re-burned the backup and now it works perfect. Interestingly enough, Nero's CD/DVD Speed shows about the same PI Error Max/Fail statistics as before, but the backup works flawlessly now.
However, after investigating this particular Pioneer model I've found that the results are sketch when used with Taiyo Yuden TG02 media (which is what I have). However, it hasn't caused me any problems yet. I think next time I will buy the slightly-more-expensive TG03 media (16X rated) or maybe even Verbatims.
I always use ImgBurn at 4X - I've found this gives the best results.