Yeah you will lose another 10 PSI through a backflow preventer. Maybe required, but a vacuum breaker would not cause friction loss.
I will look up some pump curves. The trick to getting a 5HP to stay cool at 1.7 GPM is finding one that has a good drop in horsepower when restricted with the CSV. If the amps drop by 40%-50% the motor is de-rated enough to safely pump hot water, so it takes very little cool water to keep them happy.