As I'm sure you're aware, a lot of anecdotal accounts of polyphasic sleep have suggested no loss of cognitive or creative function (after a 1-3 week adjustment period), but those are difficult to evaluate without an external metric; certain kinds of cognitive impairment can paradoxically make you feel you're thinking more clearly (c.f., "I drive better with a couple drinks in me").

Oh yes, I was well aware of that. What I did was play 20 rounds of GBrainy a day and look at my scores. (Why GBrainy? Because I didn't have a few score of comparable IQ tests handy, and it was available in Ubuntu, and was reasonably fun to play.) I forget the exact stats, but it wasn't uncommon for my score to drop by 1/3 compared to when I was sleeping normally. What seemed to be most hard-hit was working memory, which really hurt on the mental arithmetic ones.

(The obvious criticism is that I didn't actually adjust, but I don't think there's any way to prove that either way.)