My setup is a 800/5.6L IS + 5D2 + grip that comes out 6-7kg would a 12kg/26.5lbs capacity Monopod lik the Manfrotto 557B be ideal or should I look elsewhere like a Gitzo 5 series?

I don't know the 557B. Manfrotto say it's a video monopod.
If you like to have a low priced Manfrotto monopod the 681 or 681B is very good for your combo. If you like to spend more money the Gitzo GM 5541 is perfect. I use all 3 with my 800 lens. The cheap ones are rather nice and really low priced. The Gitzo is of course better but cost a lot more.

You sure are a kidder Lars I'm just wondering if I'd get less oscillation like the 5541LS you recommended.