you shoult connect the mount with the chassis, with a bracket:
like that you have
-no flex at all
-the chassis turns into a heatsink
its quickly done and it works. I have two mounts, one for a bigger motor (kb45xl), all alu in L-shape, and one made of alu and carbon for a 36mm motor, which ist definately stiffer. the one for the kb killed the teeth of the center-diff due to a minimum of flex.
aluminium is less stiff then eg. glassfiber, carbonfiber, and it is possible that it is still flexing a little.
Connected to the chassis it also reduces the forces at the center-diff mount. its made of plastic, and it could flex like that very little as well.