Looking at the specs for your burner it seems that max speed for plus(+) media is 16X but It can only reach 8X with dash(-) media.
Check out what videohelp.com has to say about your drive here.
It may be that for some reason LG has seen fit to limit burn speed for certain media codes, or the media code is not in the firmware. Either of these could explain the lower than rated burn speed (remembering - media will only ever go to 8X)
Open CD-DVD Speed (should be in the Nero Toolkit subfolder on the Start menu) and look at the Disc Info tab. That should list the supported write speeds for the currently inserted media.
As mentioned, search for a firmware update, but according to this you may not find one.....