So, why do you want to wait the data to fill buffer and to take about maybe 30min, for example, to finish the burn and let drive spin at highest RPM?
It may take you the same burn time even if you burn it at 6X, if the bottle neck is "Data Transfer Rate" from the system. And less noise and less power consumption and better write quality.
So, the best way is don't let buffer under run happen.
This happened to me, too. I use USB to connect my drive. If I burn disc above 8X then buffer under run always happen and the write speed to down to 6x eventually. I set write speed to 8X to avoid buffer under run and it work and maintain the write speed at 8X. FYI.