After working with Bill Hall last year and watching slow motion of pba stars, i can tell you Duke, Wiseman, WRW and others use relaxed fingertip grip. He showed me there videos and you can clearly see there knuckles up about an inch or so on everyone of them. It changes the point on your hand where most of the pressure comes from slightly not as much as an clt drilling but about half an inch or so. I have gained revs and accuracy from him changing my span 1/4 shorter and going to a 1 1/4 clt drilling on every ball, feels better and makes getting the ball off the hand much cleaner.
I can see why he drilled on tour for 18 years he is a great ball fitting coach watches you breaks it down to 1/60 second frames and shows you what is really happening. He is expensive and i have since went to a cheaper silver level coach but Bill Hall was worth the money to help me out gripwise.