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Can you perform a "burst rate" and a "transfer rate test" (both in the "benchmark" tab, "run test" menu) on a burnt DVDR, and post the results? This would help, maybe you have a config issue.
Something's off and must be fixed of course, as the 7173 is supposed to have the PIE/PIF reporting feature with CDSpeed, but on the other hand, I can tell you that you're not missing anything worthwhile with 7173's quality scans, they're way looney and unreliable. Scan with Benq, LiteOn or Plextor, you'll be far better off. The only NEC drive that is more or less usable for scanning is the 3540A.
EDIT You're not trying to perform this test on blank discs, are you?