Larry, are you saying that putting weight in the cockpit or another 100 lbs in the outboard would keep the stern from lifting in swells? My experience in various waters including S. Calif. PNW, Florida, etc. is that when the boat his on a plane, the boat continues to perform well, with weight aft. I don't get water in the splash well.
Now if you are saying that you never take any water into the splash well when going at 5 knots---I cannot argue against that. Except with light boat or heavy boat at rest or at displacement speed, there are situations where a sea may splash into the well, but never beyond. But I , run my boat from nothing in the cockpit with the Honda 90, and 5 gallons of gas, (Which is the way the boat is stored) to hundreds of lbs in the cockpit. The biggest difference is that I can get up to 30 mph with a light boat-and only about 25 mph with a heavy boat at sea level. I don't see much difference in the way the boat handles at slow speeds.
I would argue that the redesign of the bottom of the boat/hull in 1987 especially the reverse chine, gives more lift in the stern. I wasn't involved with C Dory's then, but my guess, is that the redesign was related to Honda expansion of all of their outboards to 4 stroke in 1985. (you know much more about that than I.) How would you redesign the boat for 200 lbs more on the transom? It is easy to say, that one might increase the inboard height of the splash well & the only place I would see where that would make a difference would be if you put several big people in the cockpit, when fishing, and then were backing down on a fish...but we don't use our boat that way. If you were to argue that that transom might need more glass, or a better tie insisted to the sides of the hull, I would agree with that, if you were increasing the HP--but we already know that the boats tend to be a bit on the squirrley side over 30 knots because of the flat bottom. The reason that many boats need more HP in this size range is because of the dead rise--difficulty getting on a plane, and more HP to drive a deep V thru the water on a plane.
It would be great to have a 100 hp outboard which only weighted 260 lbs. Maybe someday we will see that. But the way that most of us use the boats, top cruising speed is in the teens to low 20's, and 90 hp gives that at sea level with almost any load. At Powell, with about 10 to 12% loss of HP, I still can cruise in the teens, well on a plane, even with several hundred extra lbs (gear, food and water for many weeks) The reality is that we only cruised at planing speed about 25% of the time this year. Some of that was because we had to get the C Dory 25 we were taking to Antelope Pt Marina, for John and Grayle and had to cover 50 miles in just a few hours.
The other unknown is the longevity of these 4 stroke extra light motors. They may well be fine. But we will only know that after a few years of running them.