When do we know where to stop the VSUS down-tuning? Usually it'll go pink tinted in the middle of the display or you will start seeing severe dead pixels and IR. In this case high or low Vs seems to cause dead pixels - go figure.
When VSUS is tuned at the factory, what exactly the techs looking for (noise, dead pixels, coloration, etc...)? In the factory, do the tech tune the vsus voltage to the minimum till there is no apparent artifacts on screen? It is done by characterising batches of panels now, so you will usually find each panel has the same voltages printed on it if it was made at the same approx time. In general they are looking for an image with no maldischarge, full discharge on white, and minimum power consumption. The required Vs is directly proportional to panel gas pressure. The early PDPs had Vs of 165V or so, but as the gas pressure racks up, Samsung has been leading the charge in high Vs. The F8500 has a nominal Vs of 221V which is also probably part of how it maintains such high brightness.
Will going a lot below nominal vsus voltage can the damage be permanent (end vise-versa)? Low Vs cannot cause harm. Potentially extreme Vs can cause damage to the panel via overheating which can include cracking the panel. However you would have to go to extreme values to cause this. I would not advise more than about 15V more than the panel sticker.