jeally beans won't save your day. is has poor internals and is simply overpriced. what if Sony later denies update because internal wise Xperia U is superior.
Invest money in mobile with better hardware. Android 4.0 is sufficient and by the time Xperia J receives JB update, Android 5.0 will be out. And because of lack of ram as well as processing power Project Butter can't be implemented (i doubt it'll be turned on).
PS: Project Butter is mainly triple buffering hence increasing smoothness.