1/3 power (cube root) is to give lesser speed increase at greater speed orders. That is - less and less efficiency (speed increase) at greater and greater PS applies. Just look at the plot of the cube root.
NB: Cube root works well both for positive and negative ordered speeds as compared to even roots (that is why it is not square root - 1/2 power).
I will give my full analysis later, I am at it at the moment. My wild guess so far is that it is xmm2 you should tinker with to lower or increase the engine 'efficiency'.