In my area of the country, I experience the exact opposite regarding thickening and trunking up for Shimpaku. Kishu here thicken at double the rate of Itoigawa! Runner growth is around three times as large as well. Itoigawa are proving to be super finicky for me in the deep south, while Kishu seem to love it. I think the reason for this is simple: in their native habitat, Itoigawa grows where it is pretty cold and dry, while Kishu grows where it is warm and humid. An interesting point to consider regarding "best" variety for you and your region.
A sidenote: I was told by a prominent American figure that an artist in Japan famous for his various Shimpaku kept 30 or 40 different "stock strains", all with different characteristics, such as rapid root production for root grafting, rapid thickening, quick callous formation, in addition to foliage. While cutting grown Shimpaku of Itoigawa and Kishu with great foliage will thicken eventually, it's a Pretty slow process. From the little story above I've mostly abandoned it in favor of Hollywoods and Parson's which trunk up rapidly, literally 10 times faster, with the eventual goal of grafting better foliage when the trunk is finished.