For using teleconvertor, I would suggest to use on f2.8 lenses only. For f4 lenses, it affects the PQ and AF speed, maximum I would use is 1.4x on those lenses (in fact, prefer not using one). Sometime you may find cropping the picture will result in better quality. In my opinion, whether you choose 500/600/800 is a matter of price and portability. Many chings go for 500mm base on price and portability.
If you think Canon 800mm lens is expensive and heavy, Leica 800mm definitively is not your choice. Canon 800mm is lighter than 600mm due to smaller max. apetitude. Hope it helps.
