Once you have a pilot's licence, you may do whatever you want (btw, vne is not affected by exceeding masses...).
The problem comes before having the licence - at some point the student pilot needs to go solo, and if he exceeds the max load in the seat, or maximum payload, or CofG range, he needs a really dumb FI to send him off. I for my part have told prospective student pilots in such cases that there is no way.

On the max seat load, the Calif A21S doesn't have any limitations. There is just a maximum load in the cockpit which is in the range 180-200kg. I've flown it with quite massive passengers, and space is not a problem (the controls on the center console are a bit forward).

TW