1. If the top group currently pays 40% of the income tax but makes 60% of the income, giving it 40% of the tax cut disproportionately and inappropriately benefits that group.
2. If the income tax is more progressive than other taxes that are not cut, the net effect of an across the board income tax cut disproportionately benefits the top group.
Determining whether or not a particular group pays more or less than it should is a complex evaluation that ultimately involves subjective value judgments. You apparently believe the top groups pay too much. I disagree.
The top 10% pay 65% of the income tax. An accross the board tax cut would in theory not change the relative proportion of tax burden.
As for overtaxed, I believe every income category is overtaxed.