Nope, your 2MB buffer is fine. Is the problem that 40x is the highest speed Nero is offering to let you burn? If so, the problem is that the drive "thinks" that the writing quality will be too low if you burn faster than 40x on this particular type of disc. You could try updating the firmware to version 6S0F
to improve media compatibility. If this doesn't help, the only thing you can do is use a different brand of discs.