I've been using this exact combo for almost three years now (older 300mm and kenko teleconverter). Lens technique definitely helps, and handholding is almost out of the question with shutter speeds less than 1/1000 sec., which I never seem to encounter;)

I even read a lengthy discussion about the possibility of this combo yielding even MORE sharp results than without hte teleconverter, due to the recessed rear element in this lens; the coupling of the teleconverer may actually help.

I'll post a few bird pics I've taken with the combo, and describe the setup.