Hysucats run at their best with the engines almost out of the water. The engine will act like an anchor at high speed if to much of it is in the water.
We had similar problems at the recent Powerboat & RIB Show in Portsmouth due to the 300 Suzuki not being available in a shorter length.
Will get someone to advise you on the proper setup. Do you have any pictures of the boat so we can properly identify it.