The NEC and Plextor drives use different strategies in reaching 8x speed. The NEC uses Z-CLV (Constant Linear Velocity in a number of Zones) whereas the Plextor uses P-CAV (Constant Angular Velocity until maximum is reached, followed by Constant Linear Velocity for the rest of the burn).
It is easier to implement Z-CLV with a certain writing quality than P-CAV, but P-CAV is faster.
Here are some screenshots showing the difference. The first is Z-CLV writing with the NEC-3500AG the second one is P-CAV writing with Plextor PX-712A: