Probably because of the complexity of the 70-200 design, the converters behave differently depending on the focal lenght.
This was the biggest difference I found and in my opinion the most important because it shows the performance wide open
at the longest focal lenght comparable with the TC-14E (280mm) and at the longest focal lenght for the TC-17E itself (340mm).
Wide open the TC-17E works better at 340mm than at 280mm.
Stopping down 1-2 stops evens this difference (at 280mm I see more of internal flare rather than loss of resolution, indeed
I can't recognize more details in the 340mm shot despite the better contrast and higher magnification due to the longer focal lenght).