The splash (torrent) got worse when i changed from a 15 to a 25 tohatsu to the point where i had to do something about it and had a transom plate fabricated. 3mm 316 stainless steel allowed me to raise the outboard by about 60mm which eliminates the splash for that outboard. Time will tell if it damages the transom in any way but plate seems to spread the load out so hopefully will be ok.
Just need to remove the excess sealant and repaint the ply.