I'm going to be living in Japan for awhile and will be adding that new "Roam through Japan for $5 a month" to my plan. I talked to international care and they claimed I would be limited to 3G while I'm there.
This thread was pretty informative. A lot of people claim LTE is available. If that's truly the case, I'll probably burn my upgrade and get and S6 or S6 Edge. If it's not, I may just stick with my Note II. According to gsm arena my Note II has HSPA and the S6 and Edge don't. From what I've read, Softbank uses HSPA+ so having that would be better than being stuck on GSM.
Here are my sources:
Softbank frequency bands