After reading WPresto's fine test, I splurged on a 1.4xTC and the 150-450 lens. Setting up at home, I found that the lens needed no focus adjustment, with or without the TC.
I did the first real test of the combo on Jan 24 at the elephant seal rookery in Piedras Blancas, CA. As it turned out, I didn't need the TC, since the seals were very close to the viewing area. All my shooting was done with a K3, lens rigidly mounted on a monopod, SR on, AF-S spot focus, f/8.0 and 1/800 sec. The first one was at 280mm; the second at 630.
Elephant seals don't move very quickly, in no way comparable to BIF. But focus was quick and decisive, with absolutely no hunting. I'd say 90% of the shots were in focus. The one at 630mm didn't impress me until I saw that focus was on the eyes of the foreground seal. So far, so good
--I might sell my Sigma 120-400.