I am sitting rather uncomfortably on the fence at the moment. On one hand I completely agree with Robin about originality. Historic cars should be presented in the same spec as they were raced in period. On the other hand, unless you keep developing and tuning the engine and chassis, your car will slip down the order and become an also-ran.
Unfortunately we live in a competitive world where some ego driven owners/drivers think they must win, whatever it takes (Chequebook Racing). There are drivers who have no mechanical sympathy, and even less respect for fellow competitors. They will often bend the rules to the point of cheating in order to gain an unfair advantage. I also recognise that we live in an adversarial society where the spirit of our laws and any sense of justice is often trumped by clever lawyers who turn cases on some obscure point of law to produce a ridiculous result. Combine this with one of the Chequebook drivers, who has the money and connections to fight, and the FIA are hung out to dry. The FIA have their legal budgets fully committed on "modern" professional racing where countless millions are invested, and don't really understand the ethos of Historic Racing, hence the proliferation of replicas which they legitimise.
I love racing, and providing the cars are built to the same spec I don't care whether we do 50 mph or 150 mph, the best man will win on the day. However if one has an unfair advantage, it is hopeless me driving at ten tenths and going a second a lap slower than the "hot shoe" in his replica.
I don't know where all of this will go, but there are "Gentleman Events" where the organisers actively exclude those cars which don't respect the spirit of the rules. If they are able to fill their grids and provide better racing than the FIA, maybe the clone industry will eventually wither and die out.
As Robin says, Historic competition is not simply another formula in which to acquire trophies, it is a discipline apart. The trouble is that unless everyone including Jaguars, Ferraris, TVRs, Bizzarinis, Lotus Elans, Healeys, Astons etc. observe the same spirit of a discipline apart, why should Cobras be singled out to be uncompetitive? No E Type in period ever travelled so fast or revved so high, so let's start with a level playing field where the cars are representative of what ran in period.