Thanks both.
Dean, I think it depends on the teleconverter. I played with a high end OEM (sigma) converter when I was still using Nikon Gear. It produced reasonable results but there was a noticeable loss in IQ. The two converters I now have on the other hand provide exceptional performance but they have been bespoke designed by Fuji to partner a small no. of particular lenses and I think that is the point. They are not general purpose tools which try to be all things to all men (if you know what I mean).