Typically, if the disc is a high quality one, it can be burned faster. My Pioneer lets me burn 4x Panasonic DL BD-R at 10x and quality scans show it's more than fine at that speed.
As Kerry said though, it's hard to say without quality scanning to show potential issues. Verifying the burn is almost useless as you only know it can be read back. It could be almost unreadable but if the drive struggles and gets through, it will pass.
I'd use opti drive control app and do a transfer rate test. That can at least show if the drive struggles to read the disc (large speed dips during read back).