Hi Mhz
Using gravity the only way to increase the pressure to the sprinklers is to increase the pipe size. You will either need to put a booster pump in the cistern, or hook up the 5 HP submersible directly to the irrigation line. With either of these pumps a CSV will vary the flow to match the number of sprinkers you are running, and allow you to use a very small pressure tank. So yes a CSV will help you, but it must be attached to a pump.