Speaking as a student of Meibukan, the branch of Goju Ryu taught by Yagi SEnsei, from what my teacher has told me, Yamaguchi didn't actually train with Miyagi or Yagi, but was allowed to watch them training kata.
My teacher was a direct student of Yagi Sensei, back in the eighties, and is the owner of the Hombu Dojo in Australia.
Meibukan also use the one step drills, as well as Renzoku and Kakhomi(sp?) Kumite. As for Kobudo, we predominately train with the bo, but the main focus for us is kata. The kids have the eight fukyu kata and the adults have the two Gekisai kata, plus Tencho and Sanchin followed by Saifa, Shisochin, Seipai, Seisan, Kururunfa, Seiunchin and Suparempi.
Someone mentioned the katas that Yagi Sensei introduced, they were actually Chinese in origin and I'd be suprised if there is anyone other than my teacher, in the Goju circles, that knows them, including Yagi Sensei sons. In a recent magazine article, one of the sons referred to them as training drills, which suggests to me that he hasn't seen the whole katas.
Dave