Ba Wt+ is definitely not worth a $400 premium over what is already an enhanced economy product, unless the EC miles decides it for you. As others noted, WT+ seating will be closer to FI Saga class, but it's nothing special. While the shorter flights are not as good for sleeping, there will also be less need for the extra space and comfort of WT +. I have not flown it, but my impression is that Economy Comfort is closer to Club Europe in terms of space. You could thus think of FI as back to back LHR-LCA segments in CE (from a comfort perspective).

You may also wish to read the FT thread where a number of posters report getting Saga class seats with Economy Comfort tickets (and service). As you will be connecting regardless, I would save the money.