Reducing the resolution is indeed in great detail enlargement visible, compared to a 400mm telephoto cheap but the combination of 70-200 / 2.8 and 2x converter is significantly better. In typical prints up to A3 we most viewers do not notice even in direct observation.
Thanks to the electrical communication between lens and camera body of the adapter is also recognized clean and recorded in the EXIF data (focal length, aperture, lens,), in addition the correct aperture is displayed when recording.
The Vignettierungskorrekturfunktion (in DPP and when shooting with newer models) is supported by Canon for house combinations. Unfortunately there are no lens correction data for DPP.
What in the Amazon description is still missing:
In addition to the typical carton with the converter and a carrying bag and caps supplied for both sides.