Tim, I'm no physicist and totally layman at looking at charts and stuff, but I feel that chart is very much theoretical, and does not count the other possible factors.
One of them being the ability to dump E faster than the other - I've seen expert P-47 pilots like Frenchy or Lephturn quite often mentioning that in some cases, the E should not be saved and must be boldy thrown away when it is needed to be.
I don't think that refers to any extended turn fights, but in cases where two planes might start an engagement in an equal one-circle fight at speeds near 400mph, the plane which dumps E faster will often gain the advantage.
For instance, when two Spit9s engage against each other same speed, meeting in a HO merge and both turning to one side, the better pilot will often chop throttle, engage rudders, and continuously tighten his radius until he gains an advantageous position over the other pilot who's trying to turn with his throttle fully open.
I believe what Ack is saying does not necessarily represent a technical truth about plane performance, but rather empirical and tactical advantage one pilot may have over a less-skilled/experienced one - mostly in cases such as dumping E at very high speed sustained turn fights as seen when two planes spiral downwards each after another's tail, or like when one plane at high speed, tries an instantaneous turn for lining up a shot.