A key question: Can we estimate the effect of different public spending and different taxes on potential growth?

Let us assume that we can accurately estimate the effect of different government spending and taxes on potential growth. It would then be possible to : Optimise fiscal policy, by concentrating public spending on the right spending items, while avoiding at all costs tax increases that destroy potential growth; one could avoid accumulating public debt that has no positive impact on potential growth; Intelligently define the new European fiscal rules, by allowing structural fiscal deficits only when they result from increases in public spending and tax cuts that have a positive impact on potential growth, and by being able to correctly calculate fiscal solvency conditions by taking into account the impact of the structure of public spending and taxes on long-term growth.