[Trevor Booth]

Iain,
I would suggest that it may well perform better in the car than on the dyno.
Typically engine pumps are circulating pumps, dont deliver a lot of pressure but lots of flow. In closed system the inlet is "supercharged" by the oulet. (simplified description) This is why system design is important to allow flow without restriction.
I would also suggest you check with your Cyl head supplier, running on a dyno the engine was probably level, the heads/ manifold may not be able to run level but need an incline as is the norm in normal cars. The internal casting may trap steam which the std pump could not flush out but the pool pump could thru sheer volume of water pumped.
I do not profess to be an expert in water pumps, but I do know the theory behind them and have solved a few perceived "water pump" problems without doing any thing to the pump.