So you basically saying that Shikai Kenpachi> shikai Yama. Then if ever Ken manages to acquire bankai he most probably be stronger than bankai Yama too, right? That's a bit too much imo. We're talking about a person who Aizen with his hougyoku was so wary that he created a special arrancar to seal his flames just to be sure. A person who had to be tricked by Bach who could have easily killed Ichigo just to steal his bankai before he could kill him. A person whose strength was honed through thousand of years of experience. I think only Ichigo would surpass Yama and no other (without outside help like Aizen has). Kenpachi would definitely be stronger than shikai Yama but has to go bankai first.
You might be right though about Ken getting beaten even before he goes all out on his reiatsu. But it only shows that we really don't know how strong the fake Bach is. So just by basing on what he did, I would still go for shikai Yama.