Finally got a chance to use a 1.4x III extender with the 100-400L MKII and the 90D on Saturday. Certainly provides a noticeable amount of reach, but it took some getting used to only having the 27 AF point
Lucky you. Most lens/TC combos that come to f/8 on the 90D only get center and spot AF, even with all Canon gear.
It would be interesting to see if the latest versions of the Kenko Pro 300 DGX 2x work with the 90D, and allow all AF points at f/8. I've stacked a Kenko Pro DGX 1.4 with a DG 1.4, but the 90D (and the 7D2, as well, IIRC) would rack focus back and forth with those and my 400/4DO II.






this was shot on the 90D w/ my 600 f/4 III, and the ISO is actually 10,000. Forgot to add: this photo was processed w/ Topaz DeNoise AI. Lightroom was making a huge mess of things, and DN-AI rescued the shot. 





