Tunnel vision is simply your max sustainable turn when you're over your corner speed.
I think you meant "attainable" not "sustainable". Sustainable means that you can hold this turn without losing speed or alt, which is never the case for 6G turns in WWII planes.
In a knife fight, turn rate alone is not very decisive - it is the combination of turn rate and turn radius, and what is "best" is very difficult to define. Going fast enough to pull 6G (blackout) will give the best turn rate, but the turn radius will be large. One cannot get a gun solution on opponent, no matter how quickly he pulls around, if the opponent is inside his turning circle.
And here comes the "conserving E" part in the merger - many opponents who go for position on the merge will chop throttle to lose speed fast and pull a tighter (i.e. smaller radius) turn. With that they are giving up their energy in the hope of a quick victory, or that you do the same and you both end up in a slow stall fight. Against such an opponent, if you can keep your E and enter a low-G zoom, and with a good climbing plane, you can build a significant E advantage. This is a risky move because your opponent will get behind your 3/6 line initially and will attempt a difficult shot that just may connect. If you survived, they next moves are all yours.
In the P47M vs. P-38L the 38 can gamble on such a move. The P47 will pull a smaller initial turn when starting from blackout speed simply because it sheds its speed quicker - don't try to beat him in his best move. The 38 on the other hand is one of the best zooming planes and handles much better is a stall fight. A P-47 that will gamble everything on a first sharp turn and not make the shot will be in big trouble. Trying to follow a zooming/spiraling 38 down to speeds below 150mph will get it killed.
but no one guarantees that this is what the 47 will do! If he predicts your move and thinks that you completely give up initial angles to save E, he can pull a much less sharp turn, keep WEP on and zoom after you, at least close enough to shred you with 8*0.5s - In that case, chopping your throttles and going all-out of angles would have won the fight for the 38... Being unpredictable is very important as said by the previous posters.