This lens is something really exciting to test. A fixed focal length 200 mm f/2.8 lens is a dream, imagine such a lens with zoom down to 70 mm at fixed aperture f/2.8. The price of this lens is not so cheap, but the performances are really at top level, as the result of this test will show.
From the tests the main conclusions are:

• impressive to be used also at f/2.8 on a full frame camera
• for best results: at 200 mm close just 1/3 stop at f/3.2; at 70 mm close 2/3 or 1 stop at f/3.5 or f/4.0; at 200 mm the lens performance is a little better than at 70 mm in the corners
  
• diffraction effects become noticeable at f/22 (and above) when used on a full frame Canon 5D, and at f/16 (and above) when used on a APS-C Canon 60D.
• on both cameras, stars are really pinpoints in the center even at full aperture.
  

List of tests:

• aperture test, from f/2.8 to f/8 at 1/3 stops and above up to f/32 at 1 stops. Tested at 200 mm and at 70 mm. Shown the center and upper-left behavior on Capella star. Exposures and ISO were selected to maintain a constant nominal exposure, except at f/32 with the 5D.
  
• diagonal test. A longer exposure of 30 s at 400 ISO and f/2.8 shows many stars so that the behavior can be judged from corner to corner. Tested from f/2.8 to f/4 at 1/3 EV steps, increasing the ISO so that the brightness remains constant. Two tests on two cameras:
  
• Canon 5D (full frame)
• Canon 60D (APS-C)
  

Testing was made on a clear and cold night of December 2012, with automatic tracking of my observatory mount. Thanks to Giorgio Favini for loaning his lens.