How would you compare the 800 f11 to a 1200 f16? Would you use a 1.4x TC to make it 1320 f15.4? That's still not 1400 and basically f16 and you have all those extra TC elements to muddy up the image further. If you instead crop the 800 f11 by 1.5x then you have a 1200 f16.5 with only 44% of the pixels used, so you have to use software upsampling with its artifacts to get the same MP. Either way I think you would have an appreciably worse image than the 1200 f16.The f/16 on the R5 would be getting so far into diffraction limitation it would have hardly any more resolution than the 800/11 and getting very dark. At the other extreme, the 400/4 DO is performing really well on the R5, but it’s neither cheap nor very small.