I have two 840 Pros, one I kept on the earlier firmware, one I updated, for a test. Couldn't tell the difference in performance. But don't think we know everything going on inside a SSD, we don't even scratch the surface. That's why when updates are issued, they are worth your attention.
I just updated my other 840 Pro today, by coincidence, honestly. An OS drive too, completed the update in about 20 seconds, PC shutdown. Start it again and all works fine. Using it right now.
But I can see the not broken, don't fix it philosophy too. Just remember the update exists if things get worse.