Jack Douglas said:
If I buy a boat, I won't need to buy an anchor. I wouldn't want to inflict it on anyone.
Jack
When disappointment and disillusionment overcome us logic is the only solution.
Are you comparing like to like with regards the 1DX MkII and 6D images? That is, were the 6D images also shot through glass at similar temperature differentials? Atmospheric conditions can easily destroy IQ at even modest distances. Traditionally inside to outside shooting has been discouraged because of the issues air temp gradients make to images and these are exacerbated with longer distances/lenses.
If everything else is the same then don't delay, don't 'write a report' (in my experience they don't even read them) just send the camera in, it won't cost you anything except some postage and then you will know. If you are not comparing like for like then do try to do that first.
We tend to get an emotional attachment to 'our' camera, the manufacturers don't care, to them it is just another camera. They will put it on their machines and run the tests, if it is out of spec they will fix it and if it isn't they will say so.
But don't rely on testing you have done through different temperature air