Personally, I'd just spend the 1000jpy and get the class J upgrade if available. JEP+Class J would come out to 11,800jpy each way.
Not sure a JEP+8000jpy uprgade to F gives enough bang for the buck on a 1-2 hour flight, but it definitely is cheaper than booking it straight up.

It is something I'd like to try once to see how it really is though.