impossible changing burning speed after simulation

[Stefano Vesa <stefanovesa@tiscali.it>]

I'm using growisofs 6.1 and a Plextor 716A (ATAPI) burner under 
Linux.
If I simulate writing of a 16x medium, setting a lower speed, e.g. 
8x all is fine, but if I try to simulate another burn after the 
first at a faster speed, e.g 16x, the simulation speed is stuck to 
the previous 8x value.
In fact if I check the inserted medium with:
dvd+rw-mediainfo /dev/dvd
I get:
 Mounted Media:         11h, DVD-R Sequential
 Current Write Speed:   8.0x1385=11080KB/s
 Write Speed #0:        16.0x1385=22160KB/s
 Write Speed #1:        12.0x1385=16620KB/s
 Write Speed #2:        8.0x1385=11080KB/s
 Write Speed #3:        6.0x1385=8310KB/s
 Write Speed #4:        4.0x1385=5540KB/s

I.e. the current write speed, after the first simulation, remains 
fixed to the first simulation speed.
I've not found a way to reset Current Write Speed to the default 16x 
value.
The only way is to restart the system.
Is this a growisofs problem or a common firmware issue of my drive?

TIA.
Stefano