A few points to the OP.
What class of service do you pay to fly to HKG?
Here were my experiences as a former Diamond with CX.
Flying business- I got opped up quite a few times to F for free as business was oversold- note this didn't start to happen until I hit Diamond.
Flying economy- I flew what I paid for (Economy).
One other point- the upgrades with miles were not a very good deal- and other than OP UPs I never upgraded any other way (except for paying for higher COS of course).
IRROPS as a Diamond were super well handled- including putting you on another carrier (non OneWorld etc)- this is an advantage.
A key point to remember- it is really only the North American airlines that got into the routine of giving away premium cabin seats- think that the highest tier with Lufthansa gets only 3 upgrades every two years (and this is for flying 600 K Q miles), and Senator (equivalent to SE) gets a whopping 1 one way upgrade every two years (and even then it is not valid on all fare classes). Before you jump look carefully at what you will be getting- what I liked a lot about CX was their onboard service and their excellent lounges at HKG- and the odd op-up to F which was very nice. A lot more people on Asian carriers however pay for premium service (because it is much better value actually)- so typically in my experiences you end up in the class of service you pay for. The only final point is that if you are flying frequently to Latin America (you mentioned GRU in your post), you may want to look at AA and their EXP program- from YVR to GRU is much better with AA than AC because you can connect through DFW as opposed to YYZ (which is a long detour in my experiences)- and AA is super generous with upgrades and all their sticker deals.