An additional point to consider for NZ conditions is that unlike the USA, NZ does not have a CGT. So the NZ version of the "Laffer Curve" always has a leaky valve to those investments that rely on capital gains for most of its return.
So tax cuts for the wealthier will tend to lead usually to increased investment, not into taxable income producing investment but mostly into investment producing tax-free capital gains.
Any total tax return effectiveness, if there is one, from the Laffer theory is significantly reduced in the NZ tax environment?