In my experience with P2 in FCPX - it works the same as every file based camera original I've used. Disclaimer: I 've only used Canon XF, P2, AVCHD, P2, and Pro Res from a Pix 240, but yeah, it works the same as all those.
Unlikely you'll have any trouble. That said, P2 is a storage format, not an actual codec, so it's certainly possible to store data on a P2 card that any NLE might not be able to parse, but I haven't had a card in that format that FCP X can't read.
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I copy the P2 card folders into a finder folder. Then I import the finder folder copy into FCP x. I can choose or not choose which clips I want and still have a copy of the original P2 card in case something gets corrupted. Works fine.
Mac Pro, macbook pro, Imacs (i7); Canon 5D Mark III/70D, Panasonic AG-HPX170/AG-HPX250P, Canon HV40, Sony Z7U/VX2000/PD170; FCP 6 certified; FCP X write professionally for a variety of media; teach video production in L.A.