I watched Lang Lang first, and I agree that his tempo is about perfect - actually, his "Chopin rubato" is almost perfect, too. Since you've read so much about Chopin, I'm sure you've come across the idea of the Chopin rubato - it is one of the most talked about misinterpretations of Chopin's music. Keep the tempo steady with one hand, feel free to use rubato with the other, don't over-exaggerate ritards and such - very much unlike the free rubato common in his time and throughout the remainder of the Romantic period. You should be able to waltz to his waltzes, and so on. That is one of the first nitpicks I usually have with any performance of Chopin, taking too much liberty with the tempo. And just about everyone does it. It seems counter-intuitive not to.
Yes, you’re right – Lang Lang’s rubato is perfect. Better than Rubinstein, who I feel at times drags and pushes the beat too much. I really don’t like rubato so much, but I'm saying that because I only notice it in players who exaggerate it. Sometimes I hear someone trying too hard with rubato and I think, “stop messing around so much and play the damn thing.”
But players like Lang Lang are so subtle (in this piece) that you hardly notice it, but it is there. I think that is how Chopin preferred it.
Overall tempo is another tricky subject. Actually, very subjective. In my own playing, I will play a particular piece very differently on certain days depending on my mood. I know fast means fast, but many other tempo markings aren’t so clear. And I read that Chopin once taught a student how to play a certain piece, and she went home to practice it that way. When she came for her next lesson, he was in a different mood and said that the piece should go another way.
That being said, I do feel like Lang Lang's melodies were understated, and one issue of his performance brings up another question - what are your dynamic markings at measure 62-65? In Mikuli, I have fz/p at the beginning of 62, with a diminuendo beginning in the end of 63 and ending with a crescendo in the second part of 65. In Scholtz, I have no dynamic markings in 62, but the diminuendo is marked in the same place, but without a duration specified, with a p in the beginning of 65 (same crescendo in the later part of the measure).
In the Schirmer edition (editor: Joseffy) I have the exact same markings as your Mikuli. In the Paderewski edition I have only dolciss. At the end of 62 and a dim at the end of 63, and no marking at 65.
Also, Lang Lang did seem to pretty much ignore the con forza in measure 52.
He must have learned from the Paderewski edition. But did you hear how he played the LH d-flat an octave lower at measures 46 and 49? I’ve never heard other players do that before, but it sounds nice.