There are several studies that have been done regarding the effect of using a heavy bat or a bat weight (donut) when taking practice swings in the on-deck circle. The studies overwhelmingly show that swinging a bat that is more than 10%-15% heavier than your game-weight bat will actually reduce your bat speed. You can read one such study here. The section dealing with bat speed is the 4th page of the article (article page 1341) with a good summary on the next page of the article.
So what is “bat speed” and, if I have “it” will I be a good hitter? Well first, bat speed is generally what is referred to as the speed of the bat at impact. A higher bat speed, generally, will translate into a higher baseball exit speed off the bat. However, bat speed is but one factor of many on the road to becoming a good, or great, hitter. In order to be a good hitter, bat speed must be combined with things like your hand-eye coordination, your ability to recognize, and adjust to, a pitch, and your bat path.
Now it is true that if you have all the other factors listed but lack bat speed you will, most likely, either not be able to catch-up to the pitch or, if you do make contact, you will find yourself trying to beat out balls hit to the infielders. On the flipside, if you have good bat speed but lack the other factors you will hear the awesome “swoosh” of your bat as you swing nowhere near the pitch. For a really good, in depth, discussion of bat speed along with the other factors of a good hitter, CLICK HERE.