The reason you want pressure in your system is to raise the boiling point of the water, just like Guy said. You don't want the water in your motor to boil. Steam will not transfer the heat from the motor to the coolant like water will. You will get hot spots in your motor where the steam collects and your water pump won't pump steam so when the steam pocket gets to the pump no more flow, bad, very bad.
Also, take a look at the bottom of your cap. There is a check valve there to let coolant back into your system from the overflow tank when your engine cools off and the pressure drops to less than atmospheric. This is why you want an overflow tank in the first place. Without one, or if it runs dry, the system will suck air in when the pressure drops.
As far as the antifreeze mix 70/30 water/antifreeze is fine FOR CALIFORNIA ONLY. The antifreeze is there to lower the freezing point of the water so that when the temperature drops below 32F the water doesn't turn to ice, expand, and crack your block. We don't see those temps very often and a 70/30 mix should work fine all the way to about +10F
I've heard good things about the Redline water wetter. It doesn't cost that much and if it lowers your water temp it's well worth it. I've never had a high temp problem so I've never used it myself.