I have both lenses and the 1.4x (sold the 2x, not overly impressed) is excellent on either.I already have the 100-400 & 1.4TC (560mm). I was thinking of selling them for the 150-600, but I think I need to play with one first. May see if LFH have one...
The 150-600mm is a cracking bit of kit, balances on my X-H2 bodies much better than the 100-400mm and even though they weigh almost the same it 'feels' lighter. No zoom lens creep and focus is faster than the 100-400mm. Tracks better too on the X-H2S.
It is a sharp lens, but f8 does mean you have to watch the light but I have never found it to be a problem. Just something to be aware of.
The 1.4x is equally superb in both lenses (and the 70-300mm), so if you are thinking of swopping the 100-400mm, try to keep the 1.4x as it really does make a difference in extra reach.