A higher pressure radiator cap will not help you run cooler. It will allow the coolant to reach a higher tempature before boiling, but it does nothing to lower the temp of the fluid.

At sea level, water boils at 212 F. If you pressurise that water to 1 bar, the water boiling point jumps to 230 degrees. Most rad caps are .6 bar. If you go to a .9 bar or 1 bar cap, you will raise the boiling point of the fluid, but not actually lower the tempature of the coolant itself.