But in the case of trans-Pacific ops, I think would find that even with 14,000 feet of runway available at each end, the payload restriction would still make the operation non-viable with the thrust restrictions imposed.
Not really. All depends on the weather. Off a 4000 m runway (nil wind, 36°, max weight), you can go at FLEX 40 with flaps at 1+F and air conditioning off, or at Flex 37 with Config 2 and air on. But, at 37° Flex 37 actually equals TOGA, so I guess this is going to be very limiting through summer.