Yeah Zenon. I remember discussing this with you at length. I never really use the 400DOII with the 2x on the 7D2. It just sin't reliable enough. I have no problems with the same combo on the 5D4 though. The real problem with using big whites on anything other than a 1Dx or 1DxII is the voltage delivered by the battery. One might be able to make a case for favouring AF on the 5D4 or the R depending on their use but performance is really more similar than different with big whites. I find the combo works well on the R but I get faster acquisition on the 5D4. Af aside the challenges of using the combo for BIF are more fundamental. At least for me. Geese and herons and similar slow moving birds with predictable flight paths are fine but beyond that I'm reaching for an OVF.
I have not had that opportunity yet. I never really considered using the 2X for BIF. I will try both the 100-400 and DO bare and with a 1.4. I agree the EFV lag will be a bit of a challenge for erratic birds. Zone AF seems pretty decent but I have not pressed it yet. Even though I have the two "expensive" TC's I'm often torn between using them and cropping. I'm playing around with the 2X because I like a challenge.
2011. 7D and 300L F4 IS.