I think you fully understand the quality of Kau Yan when you set out the question. I have chosen Kau Yan for my kid without participating the lucky draw. It's a tough decision. But I think she will have a very good Christian based education in Kau Yan which could benefit her for lifetime. Regarding secondary school, I do not worry much. It's 6 years away from now. If she is good, she can go to good ones. I understand the headmistress will write referral letters for graduates and some go to very famous ones DGS, Ying Wa, St Stephen's, etc.
So it's all about what u want. Kau Yan has less homework, less conflicts and pressure (parents do not need to fight with their kids on homework after go home from office), small class, good moral education with well above average academics, full of love, teachers and headmistress take care of each individual student's needs very well. Of course, some parents think that without a direct secondary school is fatal.