I have a liteon 812S dual format 8x dvd burner. I have found that when burning ritek 4x dvd-r, the discs get burnt at 8x resulting in errors. Also, the discs get burnt at 8x even when i specify to burn them at 2x, 4x or 8x.
All speed selection issues are essentially firmware domain, not application program. It's firmware which ultimately determines which velocities currently mounted media can be burned at, gives the list to growisofs and growisofs picks one from this list. Even if growisofs was making up speed, firmware is not expected to burn on any other speed than one of those in the list. The list can be obtained with dvd+rw-mediainfo (as mentioned in manual page). All this means that if firmware wrongly picks too high velocity for media, it's firmware fault, hardly application. And in such case, when you can't trust firmware to pick appropriate velocity, you have the option to explicitly tell logical unit to record at lower speed. A.