Just checked with Liggy's codes and found that NEC indeed went down with the speeds for that particular media in its NEWER burner model!!! :Z
The TDK is CMC MAG E01 and rated 8x speed with the 6500 (!) but only 6x speed with the 6750 and 7550/7551. What a crap, just unbelievable. In any case, that was the correct answer and if anybody else upgrades the model of an NEC, better check with the codes first if you have a stockpile of DVDs...
PS to Liggy:
Any new FW for the 7551 coming up soon? And thumbs up for your hard work here