Good question Stephenson...
I don't think there is any empirical data supporting WaterWetter's effectiveness but people swear by it's heat dispersal capabilities.
Running at 70/30 ratio, I seriously doubt you'd see any difference at all -- the WW would, however, add a bit of corrosion protection as it has that property too.
Down here in scorching hot Houston, I've had WW in a 70/30 water to coolant ratio and found no difference in coolant temperature. With that low of coolant % though, you need all the corrosion protection you can get.
My 2 pennies...
'91 300E, 214K miles