The longitudinal stiffness of a racquet has very little influence on the power (speed) of shots hit in the center of the stringbed. (If you went to 100 stiffness that would be a different story, but the difference between RDC 50 and 70 isn't that great.) Stiffness does have an influence on power potential for off-center shots, but this is more about hoop stiffness - how well the hoop resists twisting distortion. Many modern racquets, with their relatively thick beams, can be quite stiff in the hoop but flexible longitudinally (what the RDC machine measures). I would guess that Novak's stick is pretty stiff around the hoop but flexible in the throat.
Also, the power potential of a racquet ("The percentage of impact speed returned by the bounce of the ball from the racquet.") is more important at low swing speeds. For us amateurs, the difference between a "powerful" frame and a "low-powered" frame can be pretty profound: 3-4 mph on our groundstrokes. But power potential becomes largely insignificant as swingspeed gets into the pro levels.
The pros are swinging frames around 360 swingweight, which means their power potentials are much higher than the sticks we use anyway, regardless of how stiff or flexible they are. The power potential of Djoker's stick is about 44% in the center of the stringbed. If he were to find a stick with 46% power potential (maybe a Big Bubba weighted to his specs) he would only gain about 1.5 mph on his forehand because he is swinging so fast (90mph or so).
This article explains how and why the "power" of a racquet becomes less important to shot speed as we swing faster: http://twu.tennis-warehouse.com/lear...ntribution.php