Interesting stuff, but a couple of questions for Jacko or anyone who knows a bit about these things.
1) VNAF - my understanding was that the VNAF scored well against the US in the early days because they only attacked when everything was on their side, used very good tactics (basically the same thing!) and used their slow turning ability. When the US rediscovered high-energy manoeuvring and how to use it, the k/l ratio changed dramatically.
A point of manoeuvrability being better than agility?
2) In the BoB what were the actual 109 v Spit k/l figures? I could be very wrong but I seem to recall reading that the losses for both Spit and 109 during the battle were about equal, and as the 109 was being shot down by Hurricanes as well would actually give the 109 a better k/l ratio over the Spitfire.
L'n'S does have a point, though. There certainly are a great many occasions where speed and 'straight-line' performance have carried the day over agility - 109F v. I-16 in Russia being a classic case (at the time probably the world's fastest v. most agile). And just as the Zero's manoeuvrability made it completely ascendant in the early days in the Pacific, so surely it was the Hellcat's speed, strength and dive/climb performance which defeated the Zero, rather than its agility.
But...there can't be many air battles where you can point to one decisive factor - there is too much else to consider such as pilot skill and training, tactics, defensive or offensive posture etc etc. Most outcomes will be decided by a combination of all the factors - agility being just one - and which side has the most in its favour.
If you can build a Eurofighter with rapid acceleration, long range radar and missiles, flown by well trained pilots etc etc etc, then there is certainly no harm making it manoeuvrable as well. As others have said, you never know if that might just be the decisive factor next time round...