Originally Posted by rkelley
Respectfully, closing the racquet face at contact with no other changes means that you'll just hit the ball into the net. Closing the racquet face in and of itself does not produce topspin.
Most pros hit the ball with a slightly closed racket;
Most pros swing on a nearly horizontal plane prior contact.
For Federer, hitting well above 2500 rpm of top spin, it takes a plane of roughly 15 degrees and a 11 degrees of racket face closure at contact. Of course, he does other things, but it illustrates how shallow a swing can be and yet generate tons of spin. If you swing more upward, you will get some more spin, but as you said it will indeed be only a bit more -- and you will also be sending the ball higher.
The face closure does generate more spin, ceteris paribus. The idea is that by angling your racket, you manage to make an off-center contact with the ball: you hit the upper edge of the ball, which increases the amount of energy you spend in creating spin versus accelerating the entire ball.