It's not an Imgburn issues....the 2.4X is down to your hardware/firmware. As you can see from your log 2.4 isn't supported so you'll need to choose 2X. The discs are rated for 2.4X so choosing anything above that will be overspeeding and not recommended, especially with DL projects.
If you've updated the firmware to 1.01 it didn't take...I'd try again. The firmware may fix the compatiblity issues you are seeing with the Verbatim media.
EDIT: Actually, checking the ASUS website it lists 1.00 as the current firmware for your drive.... http://support.asus.com/download/download....p&f_type=20