I have tried the 7D with the 100-400 using both the Kenco DG Pro 1.4x, which is non-reporting TC, and with the Canon 1.4x III, by taping two of the pins. The results are not good. In bright light they will focus but in poorer conditions they have been erratic. I would not recommend their use.
5DS R, 7D II, EOS-M, Powershot G3 X, 300/2.8 II, 1.4xTC III, 2xTC III, EF 1.8 STM, EF 70-200/4 IS, EF 24-105, 15-85, 100-400 II, Sigma 10-20, Sigma 150-600mm C, EOS-M 18-55, f/2 22, 11-22