I have the lens and have been using it both with film and digital and I'm very happy with the results on both. I don't have any experience with the Nikkor 300mm AFS f4, which I think has a very good reputation.
I have recently installed a chip on my 500 mm f8 Nikon mirror, this lens is notoriously difficult to focus however since the installation of the chip the camera now fires on focus. It takes a little getting use to but well worth the $30 investment in the chip and carful installation
Both give excellent pictures under good conditions (however I was never really happy with the AF fine tune on the D7000 – I don’t know the reason – I have sold this camera and use the 500p/TC-16A with fine results on D300, D700 and D800).
Both the 300mm AF-S and the 500p/TC-16A can produce sharp pictures. The 500p/TC-16A shows its age in chromatic aberration if there is high contrast, but the sharpness is excellent, and the bokeh is better than on almost any other lens.
But they are totally different creatures. I can handhold the 300mm, even with the TC-14E or TC-17E, while the 500p/TC-16A (for me anyway) requires sturdy tripod, gimbal head etc. and therefore much more load to carry.
Congratulations! The lens is as sharp as you can focus it! I owned this lens for 4 years using it on my D300 and D700. Pre-focus helps for BIF. Using the green dot on static objects also verifies in focus. Since my eyes are now 50 y/o I sold/traded this on a 300 2.8 AF and TCs. The lens may only increase in value if you take care of it. Get a good lens foot, use a sturdy mono-pod and or gimbal