Here we go again! And once again I have to state: I've done a TONNE of wildlife shooting with the 500L+2x extender stopped down to f/10. Close enough to be relevant to an 800 f/11 lens. And this on aged
Canon sensors! Mostly small birds and certainly not always in good light. People need to stop accepting received wisdom and open their minds a little.
*However* it does seem odd that the two extenders still appear to be orphaned. Even I would doubt a combination of 1600mm f/22 (although I did used to stack extenders and shoot handheld at 1400mm f/11).
Erm... what would you be shooting at night at 800mm anyway? Well there's the moon of course, in which case this lens would be ideal.