"Surely that must be authoritative?"
You'd be amazed, I've found as even a minimal researcher, the things that you'd _think_ would be authoritative ("Well, the guy worked for the company...") that turn out 50 years later to be the product of misunderstanding, faulty memory, gossip and unintentional exaggeration.
It's one of the reasons Wikipedia, for example, _can_ in many cases be useful but in many other cases is simply a now-hugely effective way of perpetuating urban legends in all sorts of fields: people who don't themselves know a great deal about the subject at hand post the information that they honestly feel is accurate, because they've heard it time and time again over the years, and as long as they have a reference they can cite (which seems to be Wiki's prime, and sole, requirement), it's accepted.
The Encyclopedia Britannica used experts, authorities, primary sources to write their articles; Wiki uses well-meaning amateurs. Big mistake, I think.
And yes, that memo from Roy Grumman to Bill Schwendler is exactly the one I referred to in my original post. It stipulated the F8F's parameters, and nobody from Grumman had yet to fly a Focke-Wulf. And Meyer is wrong about when the Grumman team went to England; it was September 1943.