In short, both HKPS and KV are very good schools. I heard class size in HKPS is a bit smaller(correct me if I am wrong).
HKPS seems less good than SC and KV is just an illusion. SC and KV has many classes, the absolute number of students going to famous primary maybe higher in KV, but this is mainly becoz the total number of graduates in KV is also much higher.
Your son is already setting in the school so well, I don't see a pressing reason for you to take risk to make a change. Unless you are particularly interested in the 催谷形式of KV.
Hope this helps.