I've basically concluded that I generally can't sleep at all on domestic flights and can maybe sleep 2-4 very restless hours on international flights. Granted, I've never flown J internationally so the flat bed might make a difference. However, I have flown domestic F and the slightly extra legroom and recline does not help me sleep at all. (On the other hand, I do seem to adjust very quickly to time changes as a result of my inability to sleep on planes.)

Based on all that, I might shell out a bit extra money for J or try to use miles for it on international flights, but I wouldn't pay multiple thousands over the Y price.