The Instantaneous turn performance envelope has bearing beyond a flat turn. A way this could be applied is flying a nose-low spiral dive at an angle that allows you to maintain 235 mph IAS and a 6g turn at the same time for maximum turn performance in a spiral dive.
Knowing this you might apply it for a variety of cases including if you in a D-11 were bounced by a higher alt & faster bogey, suck him into a nose-low manuever, use his extra speed against him while you maximize your turn performance to most efficiently gain angles to turn the tables or get angular separation.
At any rate I don't see where knowing corner velocity is useless... far from it IMHO. It's an important data point to help someone understand the bounds of the performance envelope of an aircraft. As Col. John Boyd postulated it's all about decision making and getting inside your opponents decision loop. It's an input point for developing a2a decision-making and tactics to maximize your performance in a plane.
Regarding flaps, the "lift limit" (left hand side) of the curve would move over slightly to the left with each successive notch of flaps.
I haven't tested it in the P-47D-11 but from experience you could probably still achieve velocities to acheive 6g's in a turn up to 1-2 notches of flaps which would further increase your corner turn performance (guessing in the ~215-225 mph range). Over 1-2 notches the autoretract would kick in as you exceeded the velocities for a flap setting.
Tango, XO
412th FS Braunco Mustangs