Since at least one year I had PRS+ on my 650 and since April the fabulous Zork-Edition. (By the way analogus, there is one major bug: If you update the zork-editon, all your zork-savegames get lost!
)
But I also had the known akku problems: suddenly the akku runs empty over night etc.
I followed the advice, to return to the homescreen after reading because there might be some problems with bad formated epubs and shut of the power savings. It reduced the appearance of the power losses but not completly.
Finally I returned to sonys firmware a few weeks ago (and at once missed the folder access) and ALL the powerproblems are gone (with the same ebooks on it). The reader got several times in sleepmode and shut completly down. When I started it again one oder two days later the akku remained its last state.
I know, you (the PRS+ people) tested everything and found no problem, but it works for me...sadly, because I liked PRS+ very much
Perhaps my experience helps someone.
Greetings, Scipio