What kind of boat is it? Your X-dimension is probably so low because the shallow V won't let the manufacturer mount the engine any higher. Is the cav plate dragging in the water?
To be honest, without some kind of experiences from folks who have done the same (maybe from the manufacturer?), then it's a crap shoot on how any gains/affects you will see.
The only adverse affect would be that if the X is too high, the prop will pull air when coming out of the hole and blow out. However, with the shaft 5" below the bottom, I doubt that would happen.