15hp I'd imagine is with margin and for years and years and 175+psiThe problem is if you gear it down for a 5 hp motor you will be running the pump below it's minimum speed. That can cause lubrication issues. Using pulley sizes and knowing the motor RPM (1750 or so) you can calculate the pump speed. Based on the specs I found, with 15 hp that pump is turning around 800 rpm. The minimum speed is something like 425 RPM. That means you could slow it down to almost half it's current speed. That means it would need roughly 7.5hp to run (my numbers are off the top of my head so don't take them as fact). With 5 hp you would need to go about 1/3rd as fast as the 15 hp motor. That puts you at under 300 rpm thus too slow for the pump.
I think im going to throw it on and see. Worse case it's not enough and I repower.