Folks have been doing the same kind of thing - testing activated Fi SIMs - with the Nexus 5 and with Marshmallow Previews on the Nexus 6, with varying degrees of success. Of course, since the Google Fi team has their hands full getting Fi to work on the Nexus 6, there's probably little likelihood they'll help anyone using Fi on anything except the N6 using supported builds - which is why (given the issues I already have with Fi on the N6) I haven't even considering dropping [more] money on the 2015 Moto X Pure Edition yet (especially since I'll be paying off my 2014 Moto X for a few more months, and my N6 for months after that). The 2015 Moto X Pure Edition is on my wish list, there's just too many other things going on with Fi (and Google/Android - like the upcoming M release) for me to commit to it yet.
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Alex