You don't mention what type of photography you plan to use the 300 f4 for. I use it for both my work, industrial work (destined for technical books, exhibits and manuals), and for landscapes for my wall and website. The end product needs to part of your equation. Printed to 17"x 22" and you likely won't see any difference from any of the lenses that you've mentioned.
Use of a tripod and MLU (and use of a cable release not the self timer) will make a big difference in sharpness over hand held or a monopod,
The tripod foot can be replaced, however the aftermarket ones are very expensive; making other "expensive lens" alternatives more price competitive. The cost of the RRS replacement will go a long way to paying for a better head or about 1/2 the cost of a bigger carbon tripod.
You should also look at the TC specs, the TC20E iii is listed as "auto focus not available" for the 300 f4 as is the TC17e, I believe.
I use the 300 f4 and the TC14E ii, its reasonably sharp. I don't pay too much attention to MTF graphs and other "pixel peeping" tests. Not that there's anything wrong with them, it's just that real life use is more telling. A lot can be over come especially using Clarity.
The D600 won't match MFDB, not matter which lens you end up buying. Work backwards from the end use of your images. Renting is a good way to be sure that you'll make the best choice given your needs.