I have flown in F on premium transcon flights quite a bit and the service at one time was every bit as good international F or even better. This is no longer the case; other than a better seat, the main differences between F and C on these routes is basically a bowl of soup and a tiny cup of sorbet. On a 763, sitting in C is just about as pleasant as F; on a 762, the C seats are so worn and horrid, that the F seat is that much nicer by comparison.

"Premium" transcon service was introduced by UA on this route only because AA did so first. Right now, the only premium aspect of the service is the price, which well exceeds $4K when booked in P.

Incidentally, one doesn't need to book the seat as P in order to sit there. There are often some discounted "A" fares available that offer some very significant savings. Whether or not such fares are obtainable is strictly a market-based issue, i.e. it often depends on the time of day and the location of one's connection point to/from SFO or LAX. If UA is competing with another carrier in one of these markets, the cost of that F seat often comes down considerably.