You are definitely correct that this does happen John and that basically the distance from the rider's seat back to the fairing on an 1800 is less than on the 1500.
At 5'11" I made the transition without a thought, but a pal of mine Koxx4 ( Ian) from on this site, is approximately the same hight as me and his knees almost touch the fairing.
When I follow him down the road, I have mentioned to him that it is almost hysterical that I can only see his helmet over the top of the box and passenger back rest.
The fact is that he is very long in the leg and short in the body and so his legs would be the same length as those of someone well over 6' tall.
I do know other owners of 1800's who are a bit over 6' and their knees are not closer than mine to the fairing.
As regards Mother Honda getting the bike built longer, or in two length models, I doubt that will ever happen, but a good idea might be to encourage them to create an adjustable seat for the 1800's, much as they did for the earlier models. My SOH tells me she has more room on the 1800, so maybe with an adjustable seat she could give some more room over to me.
In terms of what an owner of an 1800 can do, it is possible to get the saddle re worked, so as to cut the foam away from the lumber area for the rider, thus putting him further back on the saddle and has been mentioned by Fireshine, cruiser pegs can be purchased to give an alternative foot position that helps relieve the "squidged" feeling.