I know what 911 is on about
Different bike but i rode a XR600 with a 23l tank plus a 10kg tail pack and it was practically unrideable, removed the tail pack and it was like riding an enduro bike in comparison, loads of fun very easy to chuck about.
Can't say specifically for the 950 but you will reach a point where weight over the rear if it's high and far back (i.e. on the luggage carrier) will cause problems particularly if the front tanks are nearly empty as there's less weight on the front.
I also rode an Aprilia Pegaso in soft sand with luggage and 10l extra fuel but the rear luggage was mounted on the rear footpegs (i.e. as low and as far forward as i could get it) and the fuel was carried either side of the engine with the bottom of the cans near the bashplate and you could hardly notice the extra weight and playing in the sand was just that.