This is just speculation based on a feeling I have due to a few reported issues. It just seems that some people who have problems running the latest B7S9 firmware have also used a CVT file to convert their drives.
From information I've learned in the LiteOn conference, I know that the LiteOn drives apparently "learn" how to better optimize a burn over time. They do this by storing laser calibration information from the last four burns for each media type. There have been reports of some LiteOn drives having learned the "wrong" way, with burns getting worse over time instead of better. To this end, a utility was created to erase this information from the EPROM where it was stored (this utility was not from LiteOn, but from the codeguys), effectively resetting the drive to a time-zero state.
So it's my feeling that it's possible that the BenQ drives might do something similar. If this data happens to be saved in a data area that's designated as part of the firmware code, then it's possible that a CVT file (which is just firmware that's been extracted from the drive) could include it. Since lasers for the various 1620 drives are all slightly different in terms of their output power (and they also change during their lifetime) that calibration information could be completely wrong when burned into the firmware of another 1620 drive.
Note that this is all based on pure speculation and that it's most likely NOT happening. That's why I say that using a CVT file to upgrade your firmware is only a "small" unnecessary risk.