Just note that some drives use WOPC to check the media quality during the burning process. To do this, the drive must make use of the Burn Proof technology, pause the burn, read the disc, and resume burning. For a lot of drive manufacturers, this doesn’t cause much if any noticeable drop in quality directly related to just stopping and starting the burn.
As for quality issues directly related to buffer underruns themselves, I don’t see a great deal of quality loss until the drive reaches a pretty high write speed and continuously has to use Burn Proof. The constant starting and stopping may cause a clump of digital errors to occur on the disc that might not have occurred before, and even then, the quality of the disc shouldn’t be compromised by much.
I am not sure if the Plextor software would count the WOPC checks at instances of the use of Burn-Proof, though, but 4x shouldn’t be too much of a stress on a system unless the system is busy with many other tasks, or you are burning a lot of tiny files scattered/fragmented over 1 hard drive [or 1 large, heavily fragmented image].
Also, what media are you using to back up your data?