Even if the Laffer curve is true (see the comment by wavefunction) no one knows where the maximum revenue point is. Look at "Empirical data" in the Wikipedia article: There are many possible values for "optimal tax rate" and companies have many options to play the system, e.g. always hold back enough money that there is justification for a lower tax rate.
No, nobody knows what the exact shape of the curve is, but governments should bear in mind when screwing more cash out of companies and people in order to fund pork barrell projects.
