> The top players estimate they would need a 4 stone advantage to win a perfect player.
The branching factor for Go is so huge that I doubt anyone or anything comes close to optimal play. I heavily discount the opinions of most Go players on this topic, as they've been right about very little lately. Before AlphaGo existed, many of them thought it would be decades before a Go AI beat the best humans. Before this tournament, the vast majority of them predicted that Lee Sedol would trounce AlphaGo. And during the live commentary, I saw multiple 9 dan pros estimate that AlphaGo was behind, then gradually realize that it was winning. That's exactly what happens when you encounter a much more formidable player.
1. Coined by Eliezer Yudkowsky: https://www.facebook.com/yudkowsky/posts/10154024894449228
To be fair, before the AlphaGo paper came out, many AI researchers thought the same. I'm not in that field, though I do have more than a passing interest. If you'd asked me in 2006, I'd have said we would have robot cars before we had a computer 9dan Go professional -- and that was before all the recent progress on robot cars. My AI researcher friends mostly would have agreed with that.