Hm, it only took me 5 DVD-coasters just to find out that the 8x media can't take the 16x speed the writer can do...
And it is a bit silly you can't define the burning speed in iDVD, or even get a warning or find this described in the Help.
Finally made an image and burned it in Toast 7. Of course, after reading this thread, I now understand how to tell the Disk Utility how fast to burn.
Question remains if the Finder Burn is related to the setting in Disk Utility. Anyone knows?
Thanks.