Have replied privately to Paul, but here's the gist of what I said in case anyone is interested...and I agree with Bob, the title might be better phrased with the actual details of Paul's request!!
I've had (or have) the original 100-400 and the mk2, the mk1 and mk2 400DO and the mk1 and the mk2 300 2.8 (this is a lens that Paul also mentioned in his pms to me).
For what it's worth, the only 2 I still have are the 100-400 mk2 and the 400 DO mk2.
I use the 1 Dx with both, although it's mainly with the 100-400, and when I use the 400 DO it's generally with either a 1.4 or 2X extender (mk3).
With the extender on the DO and the 1 Dx the af, IQ etc are virtually unaffected, with the 2x af can hunt a bit more, the IQ is still excellent and suitable for heavy cropping if necessary, however I also had a 7D2 mainly for use with the DO and found with a 2x extender it was frankly terrible!!....not as bad with the 1.4 but I bought it specifically for the crop and also for a 2nd body to avoid changing lenses in dusty environments, it'\s now sold (the 7D2)and I'm using a 1D4, which, although IQ wise is fine with the 1.4 and2x extenders, af is not a patch on the 1 DX with the 2x.
I await the next body with interest!!
I swithered about selling the 300 mk2 2.8, but decided after a few months as I was always using the 400 DO there was no point in keeping it and so it went....and I haven't missed it at all, weight wise it's pretty much the same and the IQ is comparable to the 300 2.8....
Hope that helps!!
George.