[Brightcolours]

It seems like the 600mm and 800mm have the very similar optics, except that the 800mm lens has one extra (front) element and the DO groups have slightly different shapes.

Lens construction RF 600mm f11 IS STM:
https://cweb.canon.jp/eos/rf/lineup/rf600-f11/image/spec/spec-lens-construction.png

Lens construction RF 800mm f11 IS STM:
https://cweb.canon.jp/eos/rf/lineup/rf800-f11/image/spec/spec-lens-construction.png

MTF 600mm:
https://cweb.canon.jp/eos/rf/lineup/rf600-f11/image/spec/spec-mtf.png

MTF 800mm:
https://cweb.canon.jp/eos/rf/lineup/rf800-f11/image/spec/spec-mtf.png

The lenses have a fixed f11 aperture, the 600mm's IS is rated at 5 stops effectiveness, the 800mm's IS at 4 stops.

Sample images for the RF 600mm f11 IS STM (on EOS R):
https://cweb.canon.jp/eos/rf/lineup/rf600-f11/image/image-sample/image-sample-media-img-popup.jpg
https://cweb.canon.jp/eos/rf/lineup/rf600-f11/image/image-sample/image-sample-media-img-02-popup.jpg

Sample images for the RF 800mm f11 IS STM (on EOS R):
https://cweb.canon.jp/eos/rf/lineup/rf800-f11/image/image-sample/image-sample-media-img-popup.jpg
https://cweb.canon.jp/eos/rf/lineup/rf800-f11/image/image-sample/image-sample-media-img-02-popup.jpg

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Some more sample images (resized).

600mm:
https://downloads.canon.com/nw/camera/products/lenses/product-9/sample/original_sample_1.jpg
https://downloads.canon.com/nw/camera/products/lenses/product-9/sample/original_sample_2.jpg

800mm:
https://downloads.canon.com/nw/camera/products/lenses/product-10/sample/original_sample_1.jpg
https://downloads.canon.com/nw/camera/products/lenses/product-10/sample/original_sample_2.jpg

Not the best works of art, in true sample image tradition!