The speed that your burner writes to your media is controlled by your drives firmware; nero is only reporting what the drive (firmware) is telling it.
There are numerous firmware hacks out there, where you can change the write speed for a given media.
I suggest you do a search in the hardware forum for a modded firmware for your drive.
I have a liteon ldw-411s and the guys in the liteon forum have made a tool that you can change the write speeds for different media. This doesn't mean that if you change a 2x to a 4x or 8x speed that it will work. It may burn at 4x or 8x but the disc may be unreadable, thats why the speed is set in the 1st place, this modd does work well with certain media codes.
Ps Of course you won't be able to make the burner write any faster than it was designed to without using another modd if one exists.