I'm still not convinced. As lower burning speed creates better holes with deep edges as on the added illustration, also lower writing speed disks must be more resistant to ageing and easier to read with less errors.
New drives have many different and powerful error correction systems, therefore tests may not reflect the absolute truth. While some of these may be eliminated some are by design and can't be switched off for testing time.
A year ago I did some quyality testing and measurements too but results were rather contraversial and I quit.
The problem may be also, that newer DVD drives firmware or even hardware doesn't support properly lower writing speeds, as manufacturers are racing too.
Right now I don't have any need for 16x as I have time to write at 4x (approx 30 min - to have a break), also DVD burner is on every PC now, so I can use another PC, if there are more then 1 DVD to write ...