The old collar on the 70-200 was too near the zoom ring, and hindered zoom ring operation to a certain extent. I think the new one looks huge, but my guess is the new implementation is to correct the above-mentioned problem.
Comparing without any TC at 300mm f5.6 and with 2x tc at 600mm F5.6, Is there any difference in sharpness? Or Stop down to F8 with 2x tc and it will be sharp like usual?