I think since these are inboards, that the transom was probably not so robust. I would think that you would want to remove the existing coring, grind everything down and then beef it up with more coring than original, like 2"+ and definitely more glass before you cut a door out of the upper half of the transom. I think once the entire transom is really stout, then an access door would not diminish the strength of the entire structure. The part of the transom below the door is part of the entire laminated transom .... My $.02