If you're going to do that, why does it have to be CMLink specifically? Just shop for the best deal on China Mobile data if that's what you want, when you want it.
I think xiaofan's point is that if you want a single line (with a Singapore mobile number) that roams onto China Mobile, CMLink works. But if you're going to get a second data line, buy a second data line at whatever the best value is. It doesn't have to be CMLink specifically, and you don't even need to buy it Singapore.
I would add that if you already have a data roaming allotment for China, and you visit China, start with what you already have. If it works at least well enough, great, you're done. If not, THEN you can buy a second line with an additional data allotment on whichever network you prefer. In short, relax, you're fine. Cross that roaming bridge if/when you ever get there.
On top of all of that, currently we have one anecdotal report that Eight roams onto China Telecom instead of China Mobile. I trust that report is true, but it might be a one-off event. We don't know if Eight has changed roaming partners on a widespread, permanent basis. Or added a preferred roaming partner but still supports China Mobile at least in certain areas. Or something else. Let's wait for some more information, OK?