Well, I ve done some further tests with nec 3540 and the result was more odd than the previous.
I burned a TY T02 @16x using create data disc option of the nero cd speed and guess what. The disc reached 16x speed.
Then, i burned a disc again with cd speed but this time writing an ISO image file. The disc was the same (TY T02 same batch), but the speed didn't get above 14x
Then i realized that all the discs i have burned and reach 16x speed normaly were made by cd speed create data disc option (not .iso burning) This is for both nec 3540 and 3500. So i assume that master or slave has nothing to do with this. Really odd.
*Yes i have defrag my HDDs
*My PSU is only 250 watt
* I have also used DVD decrypter and Alcohol 120%. Same result