mg.metric geometry - Are all these groups CAT(0) groups?

Given a geodesic metric space $X$ together with a choice of midpoints
$m:Xtimes Xrightarrow X$ (i.e. $d(m(x,y),x)=d(m(x,y),y)=d(x,y)/2$).
Assume furthermore, that the following nonpositive curvature condition is satisfied:

$d(m(x,y),m(x,z))le frac{d(y,z)}{2}$ for all $x,y,zin X$ .
This is just a special case of the CAT(0) inequality for the "triangle" $x,y,z$.
Lets call such a space a M-space.

Such a space needn't be CAT(0), as the example $(mathbb{R}^n,d^1)$ shows, where $d^1$ is the $l^1$ metric. The choice of midpoints is given by $m(x,y)=frac{x+y}{2}$. It also needn't be unique geodesic.

But this space can be equipped with another metric, that makes it a CAT(0) space.

So my question is: Is every group, that acts properly, isometrically and cocompactly on a M-space already a CAT(0)-group?

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