The newer Nero CDSpeed has added two more reading speeds for measure/scan DVD recordable/re-writable media, 12X and 16X CAV (for SL media)...Some first initial results:
I'm surprised you don't mention the fact that when you enable 12x or 16x playback, something similar to the WOPC kicks in, which increases the overall length of time it takes to complete the scan, making it probably better to stick with 8x testing at the fastest.