Looking at the players you mention there, I'd go with the 5. ..b6 too. Just because it was played by more attacking, tactical players. With a group of thousands, it makes more sense to go into a tactical game, where it'll be easier to persuade people into a combination someone (or the alliance) sees, rather than having to convince them to play positionally... no?
Move 5. Nf3 Discussion
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Here is the chess.com database info
| 5...d5 | 838 |
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| 5...c5 | 704 |
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| 5...b6 | 137 |
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Not sure why it is not in your database (piece positioned wrong maybe?), it even has its own name.
Nimzo-Indian Defense: Normal Variation, Ragozin Variation (E51)
edit: just reread your post, you specified SuperGM... I get it now. I did find some SuperGMs (Kramnik for one) playing the same position, but out of a side-line from 3...d5
while i agree that an line with a better attack may be better against a gm should be characterized as an attacking line... at best you fake a queen side attack drawing whites forces there, and having him give up hopes for e4. Then after successfully defending and equalizing the queen side, you are left with the advantage kind side (and if you like bishops, this variation tends to keep them both for a while)... in other words, I like it, but for a different reason.
Most popular responses to 5. Nf3 in the order of the popularity are (bold moves are main lines):
5... d5
5... c5
5... b6
5... Nc6
5... (something novel)