Hey FLD , glad you have the bug -lol , I think that GSP , simply used a proven version of the tail they have in there Spitfire . I am going to tackle this because both the gray and green on this plane are easy to match .
As far as the ribs showing thru , that would be a total rebuild and that look can somewhat be duplicated with an airbrush .
Thanks to the provided pics you have shown that the current outline will allow perfact simple cuts . In addition the reason why I want to attemp this surgery is that I feel it will reduce the actual moving flying surface of the elevator .(many warbirds are elevator sensitive -so why make it larger ) As far as flutter , my #1 reason to factor for flutter is the length of the servo arm and its relationship to the control horn length at the wing surface. (if the gap distance at hinge lines is normal) . If the gab is excessive at the hindge line then flutter can occur