I dont think anyone would expect to get standing headroom in a 5.5m trailer sailer, there are no amount of design tricks to achieve this and still look accepteble, you need to get up to perhaps 7.5m and 2000kg before you can pull it off unless you can stand down in the keel, unless you have a fair bit of displacement you dont have enough boat under the water and therefore require even more freeboard. I have a boat sitting in my yard waiting for attention which i think is actually the minimum size to get 6ft headroom and still look good, its an old Lindenberg 26, the designer,Lee Creekmore used just about every trick in the book and i think came up with a good looking boat, i accept that some may disagree but it has considerable freeboard, quite a bit of displacement at 5300lbs, a nice amount of deck camber,a narrow trunk cabin which has the effect of getting the visual line of the cabinside to deck joint higher up than it would be if the cabin was wider which allows the cabin side to be lower, the cabin side is about as high as it can be, (he could probably have got away with a bit more crown to the cabintop, maybe), and to cap it all you stand on the hull, no cabinsole at all. So if the boat were any shorter it would be out of proportion.
Steve.