It isn't Nero Express that is enforcing the burn speed limitation, it is the drive itself. I believe ASUS calls their media compatibility technology "FlextraSpeed", and apparently it doesn't like that media you are feeding it, even if it is "48x certified". You may be able to disable FlextraSpeed by pressing the "Options" button on the "Final Burn Settings" dialog. Of course, doing so may result in a coaster.
Your other, and better, alternative is to find different media that your drive will burn at full speed. You should also check to see if there is a firmware update for your drive that will improve media compatibility and allow you to burn your existing media at higher speeds.