There's nothing wrong with your burner or the discs.
Speeds a media is burned at all depends on support for the media code in the drives firmware. If the firmware contains a strategy for 8X on a perticular MID then the option will be available to you in your favorite burning program.
Looking through the supported media table of the latest firmware, 2.24 and also the version I'm using, 2.23, there is no entry for TDK 003.
If there is no entry for a media in the firmware then a generic writing strategy is used which usually is less than the burners top speed and in this case certainly less than the TDK's rated speed of 16X.
A default strategy will also not burn the discs to the best standard and may produce discs with much higher errors or even ones which are completely useless.
Here is an extract from the media table using MCSE (MediaCodeSpeedEdit):
SONY D01-000 4x,2.4x
SONY D11-000 8x,6x,4x,2.4x
TDK 001-000 4x,2.4x
TDK 002-000 8x,6x,4x,2.4x
TDK 002-001 8x,6x,4x,2.4x
VDSPMSAB 001-001 4x,2.4x
VDSPMSAB 001-002 4x,2.4x
As you can see there is an entry for both TDK 001 and 002 but not 003.
Have you been able to burn any of these discs successfully with any firmware revision?