Mine on the 5DIII is +3 at 150mm and -7 at 600mm. On the 70D it is 0 at 600mm. It beats me why you sent it back for -6 and +5. I have worse results for expensive Canon primes.
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-M5 15-45, f/2 22, 11-22, Samyang 8mm f/2.8 fisheye