A powercat capable of 20 knots and above and a sailing cat are very different beasts. To make a cat able to get to 20 knots and above you have to immerse the transoms deeply, straighten the aft buttocks (the upsweep of the underwater sections at the stern) and make the
hull fine enough to get through
displacement mode restrictions.
These design criteria mean that the cat won't sail or
motor well at 5-8 knots where the drag of the "fast" shape will be much higher.
The
Gemini is a rather fat hulled
catamaran. It needs to be to carry enough payload on a relatively short length. Therefore it has a low slenderness ratio and curved aft buttocks. Hopefully the transoms are not immersed heavily or light
wind sailing will be compromised.
As many have already stated, adding twice the horsepower will not give twice the speed. This is because wave making resistance increases very quickly for fatter hulled catamarans. In the attached image the resistance curves of different hulls are shown. (Don't worry about the units - they are scaleless units but speed is on the X axis and resistance on the Y axis)
The fatter the
hull the steeper the resistance curves. I am guessing the Gemini is about 7 to 1. Its curve shows a very steep increase in resistance. Doubling the
power to allow a doubling of resistance only increases speed by a small fraction. This is what people call "hitting the wall" or similar when motoring under
power.
If you could put a few hundred horsepower on the Gemini it would eventually get to the less steep part of the graph. The weight of sufficiently powerful motors would severely compromise the whole boat.
So no. Adding more powerful motors will not make the boat much faster unless you want more power in windy or rough conditions when aerodynamic and chop derived drag increase greatly.
cheers
Phil