I cant comment on the actual effectiveness of the addatives, but the theory does stand.
'Water wetter' or similar reduces the contact angle of the water to aid 'wetting' the surfaces which it touches. go to the kitchen and put a few drops of water into a non-stick pan and you'll see the same effect (but opposite). Ok, in that case you're modifying the pan, not the water. Same can be said for a freshly waxed car
the water balls up rather than forms a sheet (which you'd expect from a fluid). You can see the same kinda effect with different car shampoos too - take a bucket of water and wash the car and you can get streaks - a good shampoo however wets the paint much better and forms a film rather than balling up. In this case you've modified the water, not the paint.
The theory with water wetter is that if you can improve the wetting of the surface you're trying to cool, you can remove heat more effectivly. I'm not sure whats inside to make this happen, but if anyones got a bottle, let me know and I'll find out!
Next potential gain is to modify the cooling agent itself so that it transfers heat better and you can also look to improve its ability to store heat. Here I'm not sure where the biggest gain is as heat transfer is somewhat negated by the fact your transfer medimum is fluid and most likely turbulant (I've got no idea how quick the coolant moves?)
Finally, you have to consider the system as a whole and what drive the thermal transit from engine to coolant. I'm assuming that this is going to be Newtonian as you've got a fresh supply of cool fluid coming from the radiator but anybody with an old car on a hot day will know that this isn't always the case
Anyway - thermal transfer is always dominated by temperature differential so cooler incoming coolant will draw more heat from the engine assuming all other factors remain constant.
So, Performance Ford's claims may be correct but they're measuring coolant temperature, not engine temperature. That is to say that the coolant is either getting rid of heat more effectivly or not taking it from the engine in the first place. The claim that cooler coolant will improve engine performance is maybe not 100% justified.