I don't think the curve describes the real trade-offs facing coercive authorities.

Obviously the point is that very high income taxes encourage productive people to do something else (eg leisure) and this diminishes tax revenue.

I agree with this.

However, lower taxes are pro-growth, and thus pro-higher-tax-base later on and eventually pro-higher-tax-revenue. So the question is not simply "what rate of taxation will maximize revenue?" because many different rates will apply depending on time preference of the coercive authorities.