. . . but not quite.
After seeing the improvement I got by using the Visoflex without peaking with the 135mm APO-Telyt, I decided—yes, again—to give the 180mm APO-Elmarit + APO-Extender 2X just one more try. And it worked!
Taking plenty of time with each frame, I shot known targets without heat shimmer but with moderate air pollution. Critically accurate focus was achieved on about 90% of shots, with the other 10%, the more distant subject matter, being at least partially off due to bad air. Shots taken at eff. f/5.6 hold up superbly at 100% magnification, but show some veiling flare at 200%; and shots taken at eff. f/8 are without veiling flare and with superb fine details that can even withstand a bit of upsampling.
Managed to get one critter along the way (see below), with the conclusion that, yes, this setup could shoot birds in a pinch. Ummm, with fingers crossed that they don’t mind standing like statues while I focus… 😜