In Toast 6 - with this Pioneer DVR-110D drive, with similar burn capability to your Sony in System Profiler, the 'status' info is:
Blank DVD+R Double Layer
If you are getting "CD-R" then the drive is not recognising the media correctly.
As your drive is Apple shipped/supported the firmware may have been updated. You could consider the 10.4.6 combo updater
http://www.apple.com/support/downloads/macosx1046comboforppc.html
You could check that you have Toast v.6.1.1
If you have had the drive a while, or the environment is dusty, you could even try cleaning the lens.
But probably the simplest thing to try is a different brand of media.
These discs are Infiniti "Professional". Other brands of media are available.
Hope this solves your problem.