I'm not saying Knights should actually be 20 times better than cataphracts, or that they should give a siege bonus due to breached walls. However, exaggeration takes something that exists and magnifies it, which means that the exaggeration of the Byzantines is how much better "Celtic" Knights are, not that they are better. When every chronicler native to the ERE and the Levant agrees that the way to beat "Frankish"/"Celtic" Knights is to either avoid melee, or use maneuvering and pre-battle logistics to ensure they aren't allowed to fight at peak perfomance, because if you do engage them in melee at their peak perfomance you will get slaughtered? Maybe "Frankish"/"Celtic" Knights shouldn't be the weakest retinue (See? I can do exaggeration as well, AFAIK Knights aren't the weakest retinue, but they aren't a strong one).
Please put the worms back in their can.