What about splitting the difference between the TC-less 300 and 300+TC2.0 by trying the TC1.4 instead? You'll still get 420mm while only losing 1 stop of light, and it should focus faster/better at f/5.6.
I use the TC1.4 with my non-VR 300 f/4, and I can't tell a difference in IQ with vs. without. I use it for taking photos of wildlife early and late in the day, and there are many times when I wish I had more light than f/5.6 gives me as-is, so I can't imagine being limited to f/8 max.
I use the TC1.4 with my non-VR 300 f/4, and I can't tell a difference in IQ with vs. without. I use it for taking photos of wildlife early and late in the day, and there are many times when I wish I had more light than f/5.6 gives me as-is, so I can't imagine being limited to f/8 max.