Yup Hikiotoshi will destroy a plastic tsuba right quick. Even with a leather one on you boken your hand can get hit. As for a tsuba on a Shinkin I was under the impression that while hand protection is part of a tsuba's existance it's primary purpose was to adjust the weight center of the sword. I wish I could remember where I read this maybe in the "Samurai Sword" I have to research it some. Anyway whoever said it intimated that the reason that tsuba vary so much in weight, thickness and design had more to do with "tuning" a particular blade to a particular person and less to do with fashion or art.
Boiled leather...harumph. Some of us haven't moved to/lived in Japan (yet, I leave next may) and have to make do with laminated leather. Oh and I was trying to be polite about Aikiken not being kenjutsu.