Well, I have actually scanned discs at 8x and 16x scan speeds and have had crazy results. Take disc X for example: I can scan disc X in a BenQ DW1620 at 8x scan speed and it will look very good. Then, I again scan disc X at 16x speed (again in the same BenQ DW1620 drive) and it can look horrible. What this exactly means I don't know. It's very perplexing at the least.
Sometimes disc X can be scanned two times in the exact same BenQ DW1620 drive (and at the same scan speed for both scans) and the results vary enough to make you wonder if it's a good burn or a marginal burn. I have had this problem with any DVD burner I have ever used; not just one particular brand. Error scanning is okay and I do in fact take the time to do many scans of my burned media. But, just what exactly a given scan means - who knows for sure? I have had scans return wonderful results on discs that wouldn't playback correctly in a home DVD player. I have also had scans say that a burn is pretty ugly on discs that played back with no problems on a home DVD player.
Error scanning is only a rough "guide" at best. I like my BenQ DW1620 more than I did any LiteOn drive when it comes to error scanning. All drives have their limitations and will not guarantee much; if anything at all. In other words, just from an error scan you could never know for sure how that particular disc will perform in a DVD player. There are just too many variables involved. Error scanning with software utilities are estimates and nothing more.