I see you are from Bloomington........I am just up the way here in the big city. Anyway, so what you are saying is that when running normal speeds, water is coming in over the back? Something is not right. I could understand it if it were coming in when you came down off plane, but not while cruising around. Where exactly does the water come in at? From the side rear portion of the transom or in the center near the motor?
I don't know if this will solve your problem, but I bet that boat could use a jack plate which will also set the motor back a little as well which won't hurt. Just make sure you have a nose cone and low water pick-up so that you don't run it out of water. Another piece you may want to try is a splash guard. Not sure where to find one, but the jet boaters will know. Good Luck, and if you are ever up this way, let me know. I am on Morse frequently.