Following up on my initial question, I just called Tachihara a few minutes ago.

They will send me their catalog in a few days, but the very helpful person on the phone (possibily Tachihara san himself) confirmed me that their superwide version could only extend up to 210 mm.

This seems to be very limiting to me and I will probably go for the 8x10 double extension instead.

Regards,
Bernard