According to IRS figures, the top 400 tax payers today pay income taxes below 16%. According to the best available figures, the top theoretical tax rate during the 1950s was over 90%, but the average tax rate paid by the top approximately 400 people was 51.2%. Obviously they used a lot of tax loopholes. Obviously they still wound up paying, in real taxes, a lot more than the very rich to today.
Of course if you refuse to believe that the real figures were reported to the IRS, you can believe any figures that you want. But it is pretty hard to reasonably come to the conclusion that the rich today are paying similar tax rates to the rich 55 years ago.
As for taxes and economic growth, it is a trivial piece of economics to conclude that collecting more in taxes immediately takes money away from the economy. However it is only slightly less trivial to conclude that the amount of damage it does is proportional to how likely that money was to get spent. Given that the poor are likely to spent more than the rich, that means that we should prefer to tax the rich more.
Therefore if you're going to levy taxes (and there is no reasonable way to avoid the need to), it is more efficient for all of us to tax the rich more heavily. Instead of taxing them less, which is what they keep telling us we need to do.
(I, along with many of the people on this site, pay a greater fraction of my income in taxes than Mitt Romney does. But is this really good public policy?)