Does anyone know if the 'golf ball' pattern actually makes a difference?
The dimples do make an improvement aerodynamically, primarily by inducing a lowering of the drag coefficient by improving the Reynold's number:
As a note, maybe 10-15% of an "eco" car's efficiency gains are from improved aerodynamics on the underside.
Remember though that increases in aerodynamic downforce also result in increases in drag, making the vehicle less efficient and and reducing vmax.
I saw a few years ago on one of the American forums someone had fitted an undertray and a crude diffuser which he said improved high-speed stability; something which is entirely plausible, but which lacked any form of quantiative testing.
Undertrays designed to reduce drag will not, in my opinion, improve high-speed stability.