A bat which has less mass, and especially which has a lower moment of inertia, may be swung faster.
Lowering the mass (and moment-of-inertia) may increase the bat swing speed, but the lower mass means that the collision between bat and ball is less effective. If the swing speed is kept constant, a heavier bat will always propel the ball faster and farther. So removing mass from the bat will actually reduce the batted-ball speed.

http://www.acs.psu.edu/drussell/bats/corkedbat.html


cork2.gif
http://www.bostonbaseball.com/whitesox/graphic/cork2.gif