Using an iPhone from Japan in Australia

I couldn't really find any posts similar to this through Google, so here goes.

I'm going on a one-month trip to Japan in November, and a whole year of study abroad next year. I figured this would be a good time to buy a new iPhone (it'll be an iPhone 6 when I go), and the difference in price only strengthened my argument.

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(click to get the larger version)

Both the Australian Apple Store and Japanese Apple Store sell the same model of GSM iPhone with the same bands. These are:

  • Model A1533 (GSM): UMTS/HSPA+/DC-HSDPA (850, 900, 1700/2100, 1900, 2100 MHz); GSM/EDGE (850, 900, 1800, 1900 MHz); LTE (Bands 1, 2, 3, 4, 5, 8, 13, 17, 19, 20, 25)

Now, here's what I'm wondering. Can I set up global roaming with Vodafone before I leave, go to Japan, buy the new iPhone, swap the SIM out, activate it in Japan (with Wi-Fi if necessary), use it, and then come back and still have access to mobile data. All the posts that I've read (in both English and Japanese) have said that SIMs do work, but none of them mention whether mobile data works.

Thanks for your help in advance. :)