For the IXD2 I think we can safely assume the hp figures are metric; the Gato figures are probably Imperial which means the metric figure is higher, but not so much higher that it would cover the difference between the power the IXD2 requires at 19.25 knots and the Gato at 20.25. So the Gato requires less propulsive power for a given speed than the IXD2, though whether that is due to better hull shape, more efficient propeller design or a combination of both I don't know.
Power requirements rise, roughly, to the third power for a given hull- so twice the speed requires no less than eight times the horsepower. Or, 1,000 hp for 10 knots becomes 8,000 hp for 20 knots. This overlooks the decrease in propeller efficiency; as propeller RPMs rise, propeller efficiency drops. On the other hand, engine efficiency rises as load increases so these effects tend to cancel each other somewhat. The net overall effect depends on individual propeller and engine characteristics. For most calculations these effects can safely be ignored.
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