I was over on the Warbirds BBS, and picked up conversation on the N1K2-J Shinden-kai.
According to Kiwi, there is a new book out in Japan on the N1K2, by Kyokuchi Sentouki, that has inflight testing data from Kawanishi itself. From what he says its a newly published book and runs about 1700 Yen ($17 if the exchange rates I got from web were correct, pretty cheap).
Shinden-kai
by Kyokuchi Sentouki
ISBN: 4-05-602064-7
Unfortunately it is in Japanese Only. But I figured that you could probably get one of the Japanese players (isn't Mitsu Japanese?) too translate the pertinent sections of the book.
Here is a link to a Japanese bookstore chain in the US that supposedly can get the book. (considering getting it myself if I can get someone to translate it for me.)
http://www.kinokuniya.com/newyork/ They have an online ordering page too, but again... the langauge barrier.
http://bookweb.kinokuniya.co.jp/guest/cgi-bin/wshosc.wb If you get a copy, would appreciate a photocopy of the data sheets

------------------
Vermillion
**MOL**, Men of Leisure,
"Real Men fly Radials, Nancy Boys fly Spitfires"