I'm struggling with the 1.4 TC + 100-400 IS mk 2 (+7D mk2); rarely a sharp focus and cannot understand how to apply micro-adjustments. I can only use it on a sunny day and the photos are NEVER pin sharp.
Are you using the mk3 converter or an older one, could make a difference. Also micro adjustment could play a part. Do not get on with the 7D2, but tried it on my 500mk2 with a 2x converter, had to get up to -18 to get a decent focus. My 5D3 did not require any adjustment.