I thought Eliezer's Lesswrong sequences might give different results. Applying your tests to those (from http://jb55.com/lesswrong/), I get 257,646 total words, 11,666 unique dictionary words, and 12,721 unique words (I'm surprised there aren't more unique words, given that the quantum physics sequence is in there). 168*257,646^.3357 = 11,010, so the sequences seem to be at about HP level.

Excellent work, by the way; thanks for the analysis.