I dont really think they are OP , the khuzaits. Ive been playing as their vassal and on 1:1 ratio our lancers ALWAYS get crushed by Empires lancers, especially their cataprachts and elite cataprachts, those things are tanks and a massive pain in the butt. I get it how khazuits can be a pain also, but maybe its because of their horse archers which indeed are very effective tool, especially since the horse archer ai is a lot better then warband and HAs will run in a pretty uniform circle around your infantry. That i conceed is very dangareous if you have infantry based army.
What id suggest if you have trouble dealing with khuzaits is to avoid open fields, its pretty logical. For a while i used exclusively horse archer (60%) and lancers (40%) in my party and i couldnt win any battle which wasnt open field. In the woods i could outnumber eneymy 2+:1 and still loose.
If open field is unavoidable, just focus on the empire cavalry and keep th infantry at minimum. As i said empire lancer are superior to khuzaits in my experiance and theyll do a fine job dispersing the horse archers