I have Mnonopod 679B as well and i used it with my MKIIN + 300 f2.8L IS + 2X and i can't tell it was not good, but it did a job, i know with larger it might not be a good option but i think it is better than handholding anyway, and if you just fold the tripod to its minimum hight then you will get more stable monopod, but as i may end up with some large lenses anytime in the future then i will never think less than 5540 and then never look back, my tripod GT5540T making my life happier and easier now even with no heavy gear yet to use on it but i am ready even for 1200 or 800 
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