What body were you using when you compared AF speeds of the 4 and 2.8? Roger from Lensrental once explained that the II series big whites use a feedback loop for which you need the 5DIII or 1Dx to take advantage, I find the f/2.8 300mm II does focus much faster on the 5DIII than on the 7D. This is most noticeable when the 2xTC III is attached.
I wouldn't say I was "comparing" as such, but simply noticing the performance of what I happened to be using at a given time. I use 7D's and have heard this assertion before about 1D's in particular being better at AF, in part due to the higher voltage battery that simply yields more power for driving the AF motor. Though it does seem odd/interesting to imply it takes a body manufactured in 2011-12 to fully utilize the advantages of a lens released in 1999.
My choice of 7D's relates to their good balance of reasonable cost in relation with very good sensor performance. There's also the good number of megapixels that gives decent room to crop without gobbling up my terabytes any faster than necessary. Also the "gripless" configuration is preferable to me, especially in the not-infrequent cases where I use a flash bracket.
The reason I went for the f/2.8 was to use it with extenders for nature photography. The f/4 is a very fine lens when bare but it takes a hit with the 1.4xTC, whereas the 2.8 doesn't, and a big hit with the 2xTC, whereas the f/2.8 is still excellent. There will be some people for whom the f/4 with 1.4xTC is good enough, but even more would prefer the good old 400m f/5.6.
Yes, I was one who preferred the 400mm 5.6 by far over the 300mm 4.0/1.4x combo (and why I was so sad to sell the 400mm). We definitely agree on the fact the 300mm 4.0 does take a performance hit with the 1.4x. I never use a 2x and can't comment on that. Results with lenses like the 135mm 2.0 or the 300mm 2.8 coupled with a 1.4x, even my 20-year-old "Mark 1" version, are virtually indistinguishable from the same lenses "solo."
And I certainly didn't mean to give any impression I criticize anybody's decision to go with a 300mm 2.8. Believe me, if I had a good financial cushion and could afford the lens without using "what's in your wallet?", I would buy one TODAY. But finance charges on such a lens would exceed what I lay out in rental in a given year. If I *had* one, I'd use it very frequently, but I have to look at how often I *need* one, which is far less.