Laffer curve
There is a point of diminishing returns from either too low a tax rate or too high a one.
That's why the Laffer Curve looks like this:
Of course, the $64,000 question is exactly where the peak of the curve lies, and that is where the arguments come in. Best way is to lower taxes and see if revenues go up. Since taxes are quite high by almost any standard (except socialist Europe), this is a pretty safe bet.