"So basically if the boat is securely anchored to the trailer (tie downs straps for the bow and stern) and using a solid support from trailer to the lower unit , you can reduce the rocking force on transom, but with a spring loaded transom saver you are only allow the rocking to happen, correct?"
IMHO, one would think so, so I'm starting to question the use of one in my mind. But let me say I'm am not a mechanical engineer.
Here's a case in point on the forces involved. Last year I was on my way down to Lake Guntersville, AL. I would do a walk-around of my rig at each rest/fuel stop which included checking the boat tie downs and the transom saver to insure everything was snug. While going through Knoxville, TN. I went on a section of elevated roadway that was very uneven; it created undulations in my rig that felt like I was riding a bucking bronco. Next thing I know a motorist pulls alongside and indicates I have a problem with my trailer, so I pull over to find my transom saver had come off the roller, drug on the roads surface, and broken the lower tang of the fork. That means the lower unit traveled at least three inches for the fork to clear the roller. This could only be a result of the motor rocking and lifting the transom saver off of the roller.
So now I use a safety strap on the transom saver that runs from one side of the fork around the frame member to the other side of the fork; hopefully that will prevent a similar incident in the future. The problem is, it’s a pain in the arse to put on while the boats live wells are draining.
Bottom line…. IMHO, I don’t think any of the devices currently on the market can prevent motor rocking, they may dampen it to some extent but not eliminate it, there’s just too much weight in the power head, and the rubber lower unit safety strap on a transom saver will give way - it’s only designed to keep the saver attached to the lower unit; and the wedge devices transfer the full force of the rocking into the transom rather than dividing the motors weight between the transom and the trailers frame.
Edit to add pic