Yeah same optics, I do like the option not having to use the RF to EF adaptor while using a TC with the "new" RF versions. Not sure that is enough to ignore the savings of picking up the EF version tho.
I used the EF 400 F2.8 III via CPS test drive a few weeks ago and I am sold.
Yes, maybe. But a test would be nice if there is a difference. As mentioned before there is no such claim on the EF 400 page. So either the RF mount makes a faster AF drive possible ...
..or only the marketing changed.
Or have I justed missed a offical statement the AF being faster with a 1 series body?
Not an ‘official statement’ per se, but mentioned on a few occasions by Chuck Westfall (Canon USA’s technical mouthpiece, sadly now deceased). For example, “In addition, the EOS-1D X achieves a higher lens motor drive speed with select L-series USM telephoto lenses than the 5D Mark III because of the 1D X’s more powerful battery pack.” (link) In other statements, he specified he is referring to EF supertele lenses.