I am quickly coming to the conclusion (with every media I have tested so far) that the best and fastest results are definitely at the rated speed, or sometimes a bit slower.
Verbatim MCC003 Made in Taiwan: only good up to 8x speed. 12 speed reads back ok, but elevated error levels.
TDK CMC MAG E01: never had any luck burning them in the pioneer at > 8x, but burn excellently in the benq at 8x.
Fiji branded TYG02: sometimes, they burn great > 8x, but the drive ussually locks the speed at 8x for some reason.
Ritek G05: Never burnt well at 12 speed on the pioneer, and don't even burn well at 8x much of the time.
TDK brand TDK 003: Burn ok at 16 speed, burn better at 12 speed.
Princo: Burn well at 4x, not brave enough to seriously overspeed this.
TDK TTG02: burns very nicely on the pioneer at 6x speed. Has PIF problems at higher speeds.
Of course, some people have media gods that give them excellent media that overspeeds well. I don't unfortunately