I just started undervolting the other day. Currently -30 across the board on my OLED with no negative results so far (even though I'm playing Baldur's Gate right now). I haven't yet tried to get more aggressive with it.

I think the general consensus is that it's possible to get up to a roughly 3-4% improvement in performance depending on how low you can get it. I think the battery benefit is in that same percentage ballpark, too, but I'm less sure on that one.