I can confirm the incredible sharpness of this lens, here's a "test" indoor shot only, but with birders parameters:
X-T1 with 400mm + 1.4 TC = 560 mm, shooting distance = 7,5 meters, indoor, so flash on, ISO 1600, f/8 (full open), 1/30 sec (hand holded) ...
It's an amazing, well built dream lens! As always the last 2-3 years coming from Fuji (e.g. 90mm f/2 !) ...
Now only the 120mm macro with OIS is missing and my DSLRs have to leave ... ;-)
For upload I had to resize the OOC Jpeg, otherwise you could clearly read the text on the box ! Look at the shimmering zone of tissues package coming from flash - you can see clearly surface of foil ...