Your example is a little flawed, in my opinion.
In your example, the Spitfire would of course be ranked higher. But that is because he fought more often, leading to more kills and pretty much a higher everything (except possibly accuracy). But, for a Spitfire pilot to get 27 kills in 45 minutes, he would have to be extraordinarily skilled. That extraordinary skill is what would give him the high rank, not the fact that he flew the Spitfire as it should be flown.
I think for your example to be a bit more fair for the hapless F4U, you'd have to add some details. For instance, in order for the Spitfire to be able to get 3 kills in 5 minutes on every sortie, there would have to be some intense action going on (and very close to his field). It is very likely that he would have numerous sorties in which he accomplished nothing more than a quick death. And remember, the equation is actually K/D+1- so if a spitfire gets 2 kills and dies, his K/D is 1.
Even in your example, the F4U would have a higher K/D (at 3.00) than the Spitfire. The Spitfire would have a K/D of 2.7. If the Spitfire pilot had ANY sorties in which he failed to score a kill, his K/D would suffer even more- and it is very likely that he would die without scoring on at least some sorties.
Basically, every 'style' of flying has inherent advantages and disadvantages. This balance is included in our scoring system.