Have friend that did that last year. His boat is a bowrider. Did not gain any speed. He did not have thru hull water pickups so had to add those also. If he had to do all over again he just would have saved the money and upgraded HP in the motors.
There was an article in either Hot Boat or Power Boat about 2 or 3 years ago were they tested two 28's with different HP and both gained speed with the lowers.