Uwe
I just got this TC and have tested it on the D800-300mm/4. I took a lot of handheld shots and very impressed with the ability to get very sharp bird images. I kept the shutter at 1/500 -1/1600 range. Very few missed shots. I was a bit surprised because I was familiar using the canon 300/4 IS +1.4x TC. The absence of IS/VR is really noticeable in the way the camera lens can be steadied in my hands--- but remarkably at the shutter speeds mentioned, I did well. I did not test my limits by using slower shutter speeds as of yet. The ability to lock focus seemed to be slightly slowed compared to the 300 without the TC but not by much. I am out of town but can try to dig out a few examples.
I have not tried on 70-200/2.8 as of yet.