Hi mrbee,
Sorry, doesnt look like the 70-300 is compatible with any teleconverter from Sigma's compatability list :-
http://www.sigma-photo.co.jp/english...q/condaitr.htm
A word of caution on the 150-500. I have one and am quite happy with it for the money, but would pay more for a better lens if I could afford. Don't expect too much from it with the aperture wide open, or fully zoomed to 500mm. It will be quite soft. Optimum sharpness is achieved at about f11 - f16 in my experience, which means you need pleny of bright light, or high ISO settings. Having said that, for now, it suits me and my pocket.
Cheers, Sie