Faster racket speed =\= faster ball speeds necessarily.
These faster rackets generally become this way by reducing their mass. With less mass you can apply less force to the ball with the same racket speed. The way I see it, the faster rackets just effect the amount of spin you can impart on the ball, not necessarily the power. We all know heavier rackets have more power or "plow through".
If anything, it's the strings to blame for increased ball speeds because it allows players to generate more spin and be able to control shots that are hit harder and harder.