I have the same drive, but I haven't tried any 16x media yet. I would guess that Verbatim 16x DVD-+R isn't in the latest A115 firmware table, so it is defaulting to a generic 4x burn. Probably will be supported for 12x burning when the next firmware comes out...
eVGA NF4 SLI mobo
Opteron 165 Dual Core 1MB cache @ 2.5ghz
2GB Mushkin DDR PC4000
2x160GB & 1x250GB 7,200RPM SATA w/NCQ
eVGA Geforce 7900 GTO 512MB PCI-e
Pioneer 111D 16x DVD burner