The question is in the title. In a recent post I read that on a merlin you could put a 300mm. Theres is not such a big difference between a 300mm/2,8 and a 800 mm/5,6 in Canon mount. But will the head able to slow down before braking to avoid vibration. Lens, body and multiplicator achieved a 6,5 kg mark. I looked at the rodeon HD VR model but i'm not sure this kind of head worth 7500 euros fully equipped. I liked to work with my hand :-) and want to try this "do it yourself" solution.