Perhaps you could machine your pump to remove the impeller (or just the blades), thereby using the standard wp pulley as an idler gear only, and install an electric pump elsewhere so you can regulate the speed of the electric pump to reduce water speed? I asume it's cavitating and the engine is overheating? Just a thought. I personally would have thought a 3SG pump would handle at least 8000 rpm all day without cavitation, but am yet to test that theory.
Either the above, or machine a larger pump pulley and smaller tensioner pulley to compensate?? (or find a longer belt)