I have had a look at that thread in the past, but I'll revisit and post a question, as I don't want to derail my own thread!

And while I'e thinking about it can someone remind me that when I'm doing comparitive test shots I have to take a picture both with AND without the tele-converter, or I'll have nothing to compare with

I used to have to manual focus with my Sigma 70-300 when I used a D40, but results were only occasionally good. If you are planning to photograph birds, good luck, the blighters move very quickly!

If your concerned at the loss of auto-focus, you might find it interesting to read-up on "Hyper-focal Distance". It can be used to give the maximum "depth-of-field", [dofs cap to those who maintain that there is no such thing]; might be handy for unpredictable wildlife (and it all is ).

Not strictly wildlife, but I took these on Saturday with my Sigma monster:

Clicky