I purchased a Sigma 1.4APO teleconverter for my f/4 300mm.  Now I can get 420mm and better get out the tripod!   I am not so steady as years before and at about 8+X in binocular terms it is not bad magnification.

Danob suggested trying the converter first since speed of autofocus can be a factor. Around here, there is nothing available to try out!  Anyway, with the lens it is mated with, autofocus seems just as quick as before and servo auto works fine, too.

The drawback is having a max of f/5.6 compared to the original f/4 without the converter. I can live with that.   

I haven't done enough "tele worthy" shots with it to post, mostly trial runs at all sorts of images.  The optics appear quite good and photos look sharp and free of any noticed abberations.  The Sigma teleconverters appear a popular item and the quality of mine tells me why.      TomDart.