I read somewhere that a person was using a non-reporting extender and taking F8 shots with decent focus. I decided to try it today. I added Kenko 1.4 DGX to my 400 f5.6. I was really impressed at how well the 5d3 handled the focus. It seemed pretty quick and very accurate.
Here are a couple shots of birds in the back yard.
1st one ISO 1600 at 100% crop
2nd one iso 3200 at 100% crop
I'm really amazed at the quality at the pixel level at these iso. These had some basic raw adjustments done in Lightroom 4.1.
By the way, by adding a 1.4 extender to a lens you use with a 7D you get about 97% of the height or width in pixels of the subject on the 5D3 with the cost one stop of light. Since the 5D3 seems much better than 1 stop of iso better than 7d it might have certain advantages in bird photography.
Thanks for confirming that this works