For the curious, Planescape made #129 on the October 25th USA Today's Best-Selling Books list (for sales from Monday the 15th through Sunday the 22nd), the first D&D release to make the weekly 150 best-sellers list this year.
Look what Amazon did! They posted about how many Stormwreck Isle boxes were sold in the last month for a particular entry of it on Amazon US. Sadly it isn't in the book ranking but I think a different WotC book is in that ranking.
So Stormwreck Isle, according to Amazon, sold about 1k copies in the last month.
It's ranked in Toys & Games and not even in the D&D group, unfortunately. Now to find that other D&D thing in toys and games and in books.