Thanks Mark for the detailed calculations, I will follow up on that later.
Thanks Russel on the friction, that will go away with two motors/wheels and give a little boost then.
You mentioned to consider the requirements, those are easy (at least for my first use case "line follower").
The surface is completely flat in simple scenario (screenshot from first 3.1m/s average video):
And I am very satisfied with the speeds achieved sofar, pretty sure that they cannot be reached because of all the turns. The last MTS run I did with this 12V motor was to take into account that the robot will have two motors/wheels sharing the load. So I moved the load into the middle of the wooden arm:
It was clear that this will reduce speed, but it did not that much (youtube video):
6.90m/s -> 5.46m/s or 24.84km/h -> 19.64km/h
Now regarding the total weight of the robot, I target for sub 70g (if using this motor) and not 200g:
4 LiPos + Arduino + L293D motor controller + soldering solution: 33g
2 motors with 2 wheels: 18g
half table tennis ball as 3rd point of contact, 2 foto transistor, 1 IR LED, board piece: 11g
So total weight of "Asurano": 62g
"Asurano" = "Asuro" like robot (see) based on Arduino "Nano"
Next MTS task is to test this 15V motor with 3 times the torque (2.16Nmm) of previous motor (15g vs. 8g weight):
Non-flat surface requirement is given by RobotChallenge Line Follower Enhanced "Seesaw" 
Hermann.