an you please tell me what distance did you set your camera when doing AFMA with your 70-200?
for the plain lens, I used the recommended 20-50x focal length rule. For 70mm I used 4.2m (60x), for 200mm I used 6.8m (34x) - that's all the room I had in my backyard.
When I attached the 1.4x, I did the AFMA at about 9.3m - 33x (I had to put the target at the end of the yard, and the camera in the house, shooting through the open door.
I am trying to honor the 50x focal lenght with all my lenses... Besides I have doubts with focal lenghts larger than 200mm.
For a 300mm ... microadjusting at 15mm seems to be a lot.