[Brightcolours]

One question, will those lenses suffer from diffraction already at  maximum aperture ?? Once stepped down how would the output look at f32 or even f64 ?

Yes of course, all lenses suffer from diffraction at maximum/any aperture. It depends on the sample frequency how you rate if the result is acceptable. Most lenses wide open suffer from worse aberrations/distortions/defects than diffraction itself, which then masks the diffraction.

In this case, I doubt that the lens(es) will have other issues wide open that are so bad that it will mask the diffraction. And the diffraction probably will be comparable to other 600mm and 800mm lenses at f11, because the DO elements sit in front of the aperture and will not shorten the distance of aperture to imaging plane much.

Canon's software (DPP and internal camera conversion) can get rid of some of the diffraction softening, by applying deconvolution algorithms.