With the caveat that in some countries the way data roaming works is all the data is routed back to your home before actually being routed to its destination - for example, if I visit Canada with a US T-Mobile SIM, all my traffic first has to travel to the US, even if I'm pinging a Canadian server.

If you end up with this sort of data configuration in China, you'll be having a bad time using it for anything performance-sensitive. Good enough for email though, I bet.