There's certainly a point where lowering tax rates cannot help much (diminishing returns). But raising tax rates from that point might have an outsize effect,

There's certainly a point where lowering tax rates cannot help much (diminishing returns). But raising tax rates from that point might have an outsize effect, depending perhaps on how much (but still assuming not that much of a change). As to growth, you're absolutely right that it's not as simple as wishing it. But the same is true for raising wages: sure, it's trivial to raise the statutory minimum, but for that to have the intended effect is a different story, unless the intended effect was more unemployment.