I couldn't wait for Christmas

[neuroanatomist]

With the 1D IV I have the following results;
24mm 0
35mm +4
50mm +4
70mm -1
I could discount the 24mm as it dropping off will not hurt much. The drop from 50mm to 70mm with a difference of 5 bothers me.

I'd be concerned, and sending it back makes sense. You could pick reasonable AFMA values, maybe W = +2 and T = 0 or +1, and use FoCal to run a single-AFMA aperture sharpness test at 50mm and 70mm, to see how they compare.

I have something similar with my 28-300L - it's around +4 at 28mm, 50mm, 100mm, and 300mm, but around -2 at 200mm. If I'd bought it new, I'd send it back (I bought it used for around 60% of the current new lens price, and while I like it, I'm not sure I like it enough to keep it, may swap it out for the 24-70 II, actually, or perhaps the 50L). As it is, looking over my EXIF with the lens, only around 10% of the shots are between 150-250mm, and it's f/5.6 at that point so AF errors aren't as critical as with an f/2.8 lens like yours.