Although many, including myself, are not fans of JFK-YVR-HKG, if you're booked in F, I'd stay with that. I've flown this many times, but if F opens up on the flight that leaves 2 hours later, book that, not J. Keep checking.
As for JFK's lounge, the BA Business Class lounge (IMHO) is better than the cramped F lounge. At least it's so large you can run laps from one end to the other.
The food is the same, no difference (at least when I am there). The desk always directs me to the business class lounge anyway (when I was Emerald) cuz they know that F lounge is not the best. There's an Elemis Lounge too, but I haven't found any showers (which they should have 'cuz I love their shampoo).
The issue really is the JFK-YVR segment in F is terrific, but it doesn't last. Upon landing in YVR, you'll be forced to wake up in the PJs, sit up, watch security run through, more passengers load, re-welcome with a new crew, schedule the meal, then take off, sleep. But it's still CX F and that's where you want to be. ^