Okay, so I can confirm that power items DO NOT pass down IVs 100% of the time. Pic related needs to be updated.

Breeding sneasals:

Mother: 31 Attack IV, holding the required power item
Father: 31 Speed IV, holding the required power item

1st - x/31/x/31/31/31
2nd - x/31/x/x/x/x/
3rd - x/x/x/x/x/31
4th - x/x/x/x/x/31

I guess the first one was lucky...