I have been experimenting with the 500mm Leica R mirror lens this winter. I have a great spot to shoot from the Juno Beach Pier near my Florida home. I used this lens on the DMR for a few months and it produced great IQ . However ISO 400 is about it for the DMR without needed to work on the noise issues and even then you lose much of the IQ.
I converted the Leica R lens to the Nikon mount using the Leitax mount. The 500mm mirror is quite easy . The big advantage is being able to use ISO 800 or 1600 and still getting high IQ.
The attached photo of the surfer required ISO 800.