My tamron 200-500mm 31A is similarly weighty. So far virtually all my pics have been with bean bags (I acquired a "pod" [Canada] with the lens and have also knocked some larger ones up on the sewing machine and filled with fine gravel) on eg walls, fence posts or the widow ledges/sills at the hides. There is no doubt that this is a reliable and effective way to go - never leave home without your bean bag with a lens like this.
I recently acquired an old slik SL67 thinking this beefy old thing would be the ticket. But my first trials with it have been disappointing. The problem is flex. Even though I am using a macro rail set up (see pic
here) to balance the lens there is sufficient yield at the connections that the lens sways with a touch.
My impression form reading around is that carbon fibre legs tend to be more rigid.
Best advice is to seek the opportunity to try out tripod + head set ups before buying. That manfrotto 55 looks rather similar to a slik able dx300 I had, which was a bit better than the tripod in the pic (an old but very practical Diawa) but not majorly so so I sold it.