CX SO is not a really a time building job, it is a place to go if you want to make it a career, from what I have been told.
As a SO, you will be the relief pilot on long haul flights. Upgrade times are 7-9 years before you make FO. SO doesn't land, only flies in the cruise phase.
I heard commuting as a SO is very difficult.
You can see why it isn't a time building job, but rather a career job. Most of my Euro friends have applied. I have not for the reasons mentioned above.
Good luck if you choose to apply.