You could do a comparison. Print off the iso chart (you can find it on the internet) on A4 or US paper, and photograph it at a distance to give the same size image as I posted. Then photograph it with your 100-400 at the same distance and upsample it at 1.5x and compare.In that case I need to copy your exact distance, camera, camerasettings and PP settings as well.
I was only joking about your 300 being a bad copy - the lens is phenomenal.
Also shooting at 2500 iso is meaningless if you are looking for details at 100% crops.
And printouts for test purpose? I dont find that to be suitable for high res comparisons.
Anyway, here is a 100% crop from my 100-400mm. raw file opened in fastone (no sharpening):
OK, use your set up and conditions and post a 100% crop from your 600mm combo and an upsampled one from your 100-400.
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