Last time I checked, the ECC setting in the Advanced dialog was ignored for BenQ DW16xx drives.
If you scan at 4x (CAV) you can trick the drive into scanning at 1 ECC by clicking on the Advanced button before scanning. The limits in the displayed graph will be shown as if 8 ECC was used.
1 ECC scanning is better than 8 ECC scanning, but you’ll find far more 8 ECC scans on BenQ DW16xx drives on these forums.
You must decide which better suits your needs.
For any other scanning speed the drive will scan at 8 ECC regardless of your setting IIRC.