I don't know about Jin being stronger than Kazuya. Going by feats alone Byran is the strongest Tekken fighter ever. Kazuya also has that nice Jack feat--and I'd like to add that 1 of the Jacks flew several meters and smashed through a wall because of the force Kazuya hit it with. So far Jin has no strength feats that I am aware of. Simply beating Kazuya 1 time does not make him stronger. Does it mean he is better &/or more powerful? That was probably Namco's intention, but strength is not the only deciding factor especially when 2 combatants are close like Kazuya and Jin.
That is all I wanted to say.