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I'm going to assume that scanning at a higher speed will be less accurate a read than at a slower speed.
Btw, most folks seem to be using 5x for scanning on NEC drives. You may want to jump into that forum for a look.
That is a huge difference in the scans, but then again: (1) you're scanning at 8x (vs. 5x) on the NEC, and (2) most people here will advise against comparing scans on different brand drives.