http://www.tsstodd.com/kor/firmware/fwdownload/ Select BD Writer Slim External, then the drive, then you should see links to pages with TS02, TS01, and TS00. Click the link for the TS02 page, and the download link should appear in English (it's an executable, .exe).
You scan at a consistent speed. Scanning speed is not linked to burning speed at all. Depending on the drive and what you're scanning, that speed could be anything. Since you see your 506CB seems to report differently at 6x and 4x, you settle on whichever produces output that makes sense. You could stick with 6x & just learn what a good disc looks like when scanning at that speed,what a mediocre disc looks like, etc. You'd have to do the same for 4x if that's the speed which produces results that make sense to you.
And since results are different from drive to drive, make sure you don't try toooooo hard to make direct comparisons; that said, you can get the general idea of how your scans look versus how the scans made by others look, and you should have an idea of when things are amiss (like when you asked your original question).