since cary hasn't responded yet I will give it a stab. No, the Cycle Stop Valve will not void the warranty on your pump. If the pipe you have now handles the pressure ok (which it apparently does ) then it will handle the back pressure from cycle stop valve. Remember the pressure still won't go higher than your high cutoff pressure of you pressure switch so about 60 psi in the pipe downstream of the pressure switch. For a 1 hp pump the CSV125-1 would be my choice and psi of 50 or so. You could also get the CSV1a if you want to adjust the pressure as low as 15lbs I believe but I would only get this one if putting in new tank pressure switch etc just due to extra cost.
I would suggest however, that if you use the irrigation a lot and don't need a high pressure for that you could use two CYCLE stop valves and systems with lower pressure going to the irrigation system to save energy.
