Step Turn,
I believe that there is an STC for the Lycoming engine on a Chipmunk. In which case, it's airworthiness would be documented.
I still wonder about Art Scholl's Chipmunk, with its major airframe modifications, particularly the clipped-wing and full-span ailerons.
I don't agree that aircraft are flown "in public" at speeds they have never before achieved.
I agree with you. Much as I love seeing warbirds flown fast, an air race is not the place for test flying!
The thing that really surprised me about Galloping Ghost was that they had disabled one trim tab, presumably with the thought of reducing trim drag, without realizing that the other tab would now have to be deflected over larger angles for a given speed, thereby increasing trim drag and setting up the potential for flutter.