16x DVD burns at 4x only

I just bought a couple of Verbatim 16 DVD+R spindles. To my surprise, they burn only at 2.4x and 4x on a NEC 2510. This is a 8x burner.

Burning Verbatims 8x (or Fuji's) DVD+R's have an expected 8x speed.

Is this 4x speed, because the 16x DVD's have a limited 'speed-spectre' ?
 
Since that is an older drive does it have the latest available firmware? older firmware may not have the burn strategy for newer DVD Disc's.
 
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