Freehills,
I think you are correct about that. The -8I struggles in the same way, but to a lesser extent since the upper deck is narrower and shorter. Doing the same math on the -8I reveals that it carries about 26-27% revenue as a percentage of MZFW. It is smack dab in the middle between the 777 and the A380, right where you would expect it to be.
So the question becomes twofold: Does CX want to sacrifice big twin efficiency for the prestige / product offering associated with the quads? And does CX take an incremental / slightly less glamorous step with the -8I? Or does it make a larger / more risky / more glamorous A380 investment?
Knowing the market and the Swires, it's not hard to figure out that it will be the -8I or nothing.