Of course getting hard data in economics is close to impossible since you can't run controlled experiments. The closest you can get is looking at historical data such as comparing what happened after Clinton's 93 tax increase and after Bush's 01 and 03 tax cuts. Or comparing similar countries with different tax rates. But there are just too many variables to really isolate the data and reach a firm conclusion. I do remember reading about some economist's study that tried to look at the data and came to the semi-conclusion that it doesn't seem that tax rates have much an effect on GDP at all (sorry I can't find source right now).