I was just reading a piece by supply side economist Alan Reynolds from a month ago. It contained some rather entertaining quantitative reasoning:
"If so, cutting someone's marginal tax from 40 percent to 30 percent would typically result in about 16 percent more income being reported. With 16 percent more income and a 10 percent lower tax rates, revenues would certainly not go down."
I think it's safe to let that one speak for itself.