Considering purchasing the R6 but have read that my ef400mm f2.8 is L mark1 will not work with it. I.e. I wouldn't get 12 fps in mechanical shutter, is this true? And would the eye autofocus be effected too? Any advice would be appreciated..
No first hand knowledge here with that combo, but your looking at a almost 30 year old aperture motor technology with the Mk1, seems like early generation ones seem to have a issue with their limitations and with power consumption based on this page and the chart. https://support.usa.canon.com/kb/index?page=content&id=ART177431&cat=PRODUCTS&actp=LIST In any even the focus and aperture motor are different motors and while older lenses are not as good performers as their new counter parts I would be surprised if the r6 AF system did at least perform as well or better then the older camera bodies you have used with that lens.
i would ask myself what I think my gear will look like 5 years from now if I was in your position. If you are pretty sure you are going mirrorless vs staying with DSLRs I think you have our answer. But of course I do not know your full picture, we only know the camera lens combo you mentioned as part of your consideration. We do not know your gear, how you use it, or anything of that sort so the suggestion is pretty high level.
