I had a look for the term and actually this is used in Astro-Physics for describing the centre of mass between planetary objects - I know the RRR is a star, but .....
But seriously, what you are refering to (schwerpunkt) is Centre-of-Gravity.
To my mind, the difference bettwen the lying and standing servo position is minimal and I doubt most people would be able to tell the difference.
The lying servo will have a lower c-o-g, but with the standing servo, the weight will be more central to the middle point of the chassis (should help for faster side-to-side transitions).
I have the Evo kit built into my RRR-WCE, but I left the original upperdeck to have the lying servo arrangement.
But I believe another advantage of the standing-servo is the direct linkage to the brake rod - probably the brakes work more efficeintly in this arrangement (aber wer bremst verliert
I'm going to stick with the lying-servo arrangement, as I like the positioning of the receiver.