I've never had any issues using Japanese boards but then again I use my left hand exclusively for space and shift, the only thing that does bug me a little bit is the small backspace :P however when I use it I never really have any issues.
mine:
Show Image
(http://img.photobucket.com/albums/v104/enthauptet/bin/C1b.jpg)
Glad to hear that you don't have issues with the layout. I've been thinking about how I currently use the space bar and I find that I use both of my thumbs on it but mostly the right thumb. And both of my thumbs rest or naturally hit the space bar at the gap on either side of the B key. So it may work for me as well. But I may have issues with the enter key I've noticed that I hit it on the left side of the key not the middle so I would have to learn to stretch over a little to hit it on the Japanese layout. That should be doable.
I've given up trying to find a Japanese board, I may have to turn to those nice Koreans for help:
Show Image
(http://geekhack.org/attachment.php?attachmentid=4019)
Imagine the 2 keys beside the space bar as Fn keys.
And I can get one from Korea (http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=130322578968) with free shipping. The total is 25% less than shipping alone from the U.S.
Crazy world.
I've got three of them in the process of being shipped from Japan. One should ship this week. The other two may be another week or more before they ship. What kind of Japanese board are you looking for? HHKB Pro or something else. The HHKB Pro Japanese layout is going for 22k-24k yen new but I've seen some on Yahoo Japan for less used.
EDIT: Forgot to add what are you using for generating the above layout. If I did not know better that looks like it came from xkeycaps but that would include the scan codes.