The thermostat for hot summer driving is none. The increased volume of flow simply allows better cooling IF your water pump belt is not slipping and your radiator core is clean. And no, you don't need to block the bypass hose. But if you are going to use a thermostat consider that the thermostat's function is to block flow until a minimum temperature is reached. If your car, on its best temp days, runs above 180 with a 180 thermostat, using a 160 thermostat will still let it run above 180 in the same ambient conditions. How much heat the water can absorb on one side and how much heat the radiator can disperse on the other (including maximizing air flow and maximizing conduction from water to metal and metal to water) are really what counts.
Bob