Thinking of getting the Sigma 80 to 400 os

I have been looking at the same lens since it is about $400 less than the Nikon 80-400VR. I don't think you would be able to effectively use a TC on this lens since on the D-70 you loose autofocus below f5.6. I am using a 70-300ED and want longer without having to use a tripod which is why I am looking at the VR/OS lenses.
Bob