The best extender is a crop camera. It gives y deeper FOV (if y want), and by more pixel desity y get more magnification, without accepting IQ reduction by additional glass.
I have expierience with the old 300 2.8 IS, a 2xiii and a 50d. With this combination already its really difficult to overcome camera (and tripod and lens foot) shake, means its better than cropping only under ideal conditions.
Other than from a technical point of view, i dont see much use for a manual Focus, 840mmm f8.0 combination on a crop camera, maybe exept under very bright lighting conditions. Normally, cropping will give better Image quality.
For the frequently seen moon shots, with normal techique athmospheric limitations may be more relevant than all other sources of IQ reduction.