It will probably work well with the 50-140mm, but I'm not convinced when it comes to the 100-400mm. I got an early PP sample a few weeks ago, but I'm still waiting for a proper firmware update for lenses to actually support it electronically.
They promised more compatible lenses down the road, so one can wonder which ones. The XF90mmF2 and XF60mmF2.4 come to mind, but it doesn't fit there (I checked), so IMHO, this leaves the rumored 80mmF2.8 Macro. Especially since Fuji has already promised that the converters would work with the now cancelled 120mmF2.8 Macro.