Earlier this week (just for grins and field readiness) I cut a 45minute P2 piece working solely off a Western Digital My Passport 1TB USB 3.0 drive and had no problems at all. I was quite surprised. I didn’t cut it in FCPX though. PPro. But still, I was quite pleased with the performance. RGB Curves and Fast Color Corrector on every shot plus about 40 jpeg stills. Rendered all the audio out as .aiff and mixed and sweetened in Audition as well. I cached media files to LaCie Quadro USB 2.0, but I rendered out the final .MOV to the WD right next to the source footage. No problems at all.
I did this on a new 15″ MPB Retina. 512 SSD and 16 gig of ram. I have a 7200 rpm Seagate drive on a Thunderbolt adapter and the WD returns slightly better speeds on a Black Magic test.
Your mileage may vary of course, but USB 3.0 worked pretty seamless for me.
It’s a solid option from what I can tell.