If you do the theoretical calculations assuming perfect collision, a free standing racket will reflect the ball at around 70% of the incoming speed (assuming a 12oz racket and a 2oz ball). Whereas if the racket is moving towards the ball at a given speed, almost twice that speed will be added to the reflected ball. By this logic, I would think that since loosening the grip increases racket head speed so much with relatively little effort, it is not worth trading it off for blocking the ball with a tight grip, at least for ground strokes and definitely for serves/overheads (executed with the right technique, of course).
I read somewhere "Relaxed wrist, firm fingers" and iirc it was Jim McLennan. It sounds confusing at first, but then I found the below which clarified things somewhat. It references the Spanish Tennis Federation's concept of "Strong fingers, loose wrist".