Hiya, I think you made the right choice with going with the 300mm f/2.8L IS II. I had the 500mm f/4L IS for about 2 years and it was good, but the 300mm f/2.8L II has a clear advantage in IQ. The new IS is awesome and there is a new mode 3. The AF speed is stellar with the new Mark III teleconverters. I use the 2x III on the 300mm and the images are super sharp at f/5.6 (wide open). Pared with a crop sensor camera and you have a beastly setup.+++1
Needless to say...I also picked up a 600mm f/4L IS II which mostly lives with a 1.4x III or 2x III on it. It works great too, but I still think the 300mm f/2.8L IS II is the sharpest. I don't think I will ever get rid of the 300mm unless for the replacement version.
Best of luck and happy shooting!
Thank you sir for this enlightening notes from your experiences from using the EF300/2.8L IS II USM! The combination of EF300 II with both Extenders III and with the addition of the remarkable sharp EF600mm f/4L IS II USM is so far the ultimate combination to capture birds and wildlife.
(As a little side note - of course we might wait and looking for, if Canon will upgrade - the old EF800/5.6 I to maybe a optimized new Canon EF800mm f/5.6L IS II USM? ... but today these two new version II lenses mentioned above, is maybe the best options to bring outdoors, with you for the best results also together with the both Extenders version III!
Wishing you all the very Best and many many great keepers!!
Spring Greetings from Sweden!
(PS. BTW, I have recently placed an order for the new EF300/2.8L II)