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.
We know the Tammy is very good at F8@600. And also very good at 400mm wide open. Question is, Whats best: upsampled 400mm wide open vs 600mm wide open. Please show us!
I cant see a comparison between my lenses (upsampled 100-400 vs 300mk2 +2xtc) can contribute anything. What I am saying is that my 100-400@400 f5.6 looks much sharper than your tammy @600mm f6.3 examples.
I agree the Tammy looks great at F8. But for me, f8 is not very useful.
However, if tammy @ 600f6.3 gives more details in a 100%crop than an upsampled tammy 400f6.3, it start to be very intersting. Hope you can make the comparison!