OK, all silliness aside, 5 years later and _none_ of the computers treated with Plexus have yellowed even a little. I still do my best to keep them out of the sun, dry and away from temperature extremes, etc.
Only downside is the toxic chemical blast, so I use it rather sparingly, with no one else home and open windows. :-/ Still, seems effective for keeping cases looking new, especially after "retrobrighting" since those plastics may be more prone to yellowing again.