It is very easy to overcook jumbo prawns and jumbo shrimp. There is no difference between prawns vs shrimps in this regard! You want to serve the grilled prawns right at the moment when the jumbo prawns are just completely cooked and not leave the prawns cooking longer and longer. When jumbo shrimps and jumbo prawns are perfectly cooked they are still full of moisture and are plump and succulent and have just turned opaque.
The ease with which you can over-cook jumbo prawns is the down side of the fact that they are so incredibly quick and easy to cook. Just keep your eye on them so you don't ruin them by over-cooking the prawns on the grill!