I'm been thinking about x2 TC III for my 400mm f2.8 IS II. It would be nice to have 800mm range for wildlife. I'm little concern about IQ loss Don´t be. It is still great. Take it to a store and try it out. You will be very pleased. The attached image is a 100% crop
5DIII, 1/1000s, f5.6, ISO400, handheld.
One of the best bird photographers I know uses the 400/2.8 II + 2xTC to get spectacular photos.
5D IV, 5DS R, 7D II, EOS-M, Powershot G3 X, 400mm DO II, 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