I was reading the HKIS post and i couldn't help but laugh. Especially the one about GSIS and CIS being like Man U vs a varsity HS soccer team - that was a favorite.
I don't go to any of these schools. I teach kids from all three of these schools. This is what I would suggest you do. Ask the school the exit stats. Specifically, how many kids out of how many in the total class went to which top universities. I don't know about HKIS, but I know GSIS in the last two years had crazy stats, with more than 15% of its class going to an Ivy, MIT/Stanford or Oxbridge. Compare that to a top US boarding school, and its still up there (although not in the top 10).
Don't think HKIS has those comparable stats. What I also say is HKIS has lots of legacy kids who get in because of their parents alumni links (I know, i have taught kids that were easily not HYP material but got into one of those three because their mom and dads went there and they are prominent people - although GSIS has its share too).
I ain't a HKIS hater, they got great students too. However, I do think CIS and GSIS are way better at training kids consistently. Much more so than HKIS, which has its share of stars and duds. Again. I am sure there are duds at CIS/GSIS as well, but my personal experience with teaching a whole bunch of them from these various schools is that HKIS kids are (generally) more "confident" about their self-worth while the other two schools' kids just grind it out and do as best as they can - no intrinsic belief that they are entitled to anything without working hard for it.