Donut bokeh is liked by some people for "artistic effect" but to me it always looks just plain ugly.I wonder, if Canon can manage the typical trade offs of such a catadioptric design.
(bad contrast, low resolution to the edges, "halo" in the center, bad bokeh).
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It's inherent with catadioptric designs, AFAIK there's no way to hide or minimise the effect.
Personally I'd *much* rather Canon brought out a lightweight "telescopic" 800mm F8, along similar lines to the RF 600mm F11 and RF 800mm F11 designs. Such a lens would probably be cheaper and lighter than this catadioptric 800mm F5.6, it wouldn't have ugly bokeh, and it would be far more useful for bird photography.
It would be interesting to know which of these designs would appeal most to sports photographers - 800mm F5.6 catadioptric, or 800mm F8 telescopic?