Although good close-ups, I can discern some lessening of sharpness in these pics as compared to the pictures I took earlier today @ 400mm [no extender being used]. The lack of bright sunlight is also a factor–skies are very overcast. The lighting is all ambient.
I think there are shots, other than lunar [some stacked and sharpened] and solar photographs where an extender is useful–particularly as compared to the cost of an 800mm prime lens. Anyway, here you have some of my pics for comparison.