Well, my understanding is the following:
If the drive is not affected, there is no reason to run the tool right now. Your're drive's read performance won't suffer in the future, because the new new firmware takes care of that(I guess that they expect to thrust them about it. Only time will tell us if they are honest, but I don't think that they are that stupid to risk their reputation because of this: they will need to sell the new 850 evo's). The new firmware will also be available using Samsung's magician software. They also stated that all new factory 840 evo's will be shipped with this new firmware.
Source:
http://www.custompcreview.com/articles/samsung-840-evo-performance-fix-available-tomorrow/22161/
Now the issue is the vanilla 840. There are people reporting that this drive has the same issue, but samsung says no ... maybe because it is a phased out product and they just don't care about it anymore? If this is the case, then it is just very bad PR and selling future products may be a problem for them.