The guys over at CDRInfo have tested some Verbatim and Taiyo Yuden 16x DVD media burned at 20x on a handful of writers and compared the results with a DaTARIUS scanner. They also tested readability, but unfortunately only in the drive that did the burning so the readability results cannot be compared across drives.
The article/review is here:
They conclude that:
- 20x writing affects burn quality negatively
- Taiyo Yuden 16x DVD+R (YUDEN000 T03) performed best of the tested media
- Pioneer DVR-215 performed best of the tested drives
The review and your own experiences with 20x writing can be discussed in this thread.