Here is a sample of performance of the new 100-400 with the 1.4 TC
I would expect even better if taken from a solid rest or tripod (this was in a rocking boat)





5D MKIV
Canon 100-400mm Version II + 1.4 TC @ 560mm ... hand held from a boat
1/1600
f/9.0
iso 320
Exposure bias -1 EV