I have used the Sigma 120-300 with a tc. it is best done with a tripod, your shutter speed will slow down. I think a 1.4 is plenty. I just got a used 300f/4 for my d800 from B&H used, it was 975 and listed as a nine but it looks new to me. If you get a used one be sure to check the inside of the lens surface for dust--the back of this lens is not covered. I have only done test shots so far but the focus is very good. This is the only autofocus lens in my kit.I plan on taking to Glacier next summer.