That's a good question - last week I tried some birds in flight (not really my main interest but thought I'd try something slightly different) and was using the 5D3 with 300mm f2.8 IS and Kenko DG 300 1.4x TC, 9-point expansion, silent shutter continuous drive and AF tracking mode 5. I didn't take too many shots but didn't get any keepers. I put this down to:
1. Need more practice - I normally do portraits/city/landscape type stuff so flying things are harder for me.
2. Using the 1.4x TC maybe slows the communication to the lens meaning it isn't responding quickly enough using AF mode 5. I used the same body/lens/TC at an airshow and AF mode 2 after reading about airshow photography on another site. Whilst I did get some very good shots, overall I wasn't impressed by that tracking mode for aircraft (prop and jet).
3. Uh....something less obvious. ???
Hopefully our friends on this site will share their wisdom soon!