yama is powerful, but that's mostly with his bankai. now i'm not saying that he is weak without it, just that against high tier opponents he needs his bankai. without it, we saw how he got one shotted by juha bach. now ichibe, we see he is outclassing juha without even using his shikai. at the end of the last chapter we see him activate his shikai for the first time. that means everything to that point was done with a paint brush that acts as a sealed zanpakuto, hokudo, and kido. plus you also have to remember what a fight between shinigami is as explained by aizen. a fight between shinigami is a fight between spiritual pressures. which for the most part extends to other beings who fight with reiatsu.
so, just going off of that, it's more than possible that ichibe would be able to cut yama's power in half. yama wouldn't be able to do jack $hit against that. i think ichibe outclasses yama. there is a reason this guy is head of the royal guard.