I understand the reasoning behind this if you didn't already have the 300mm and 1.4X. Surely if you're not using the flexibility of the zoom, the prime would be a better option? Perhaps you don't want to be swapping lenses between bodies, just interested.
I don't have the 1.4x. I have the 2.0x but the quality is meh. I've been thinking about getting the 1.4x but didn't see logic in spending 500$ more to make the lens shoot as if she costs 500$ less.
I use the Tamron at 420mm on the D7200 since I can crop that to 100% and still get a decent shot. The 300mm I shoot solely on the D810 since I have some pixel advantage and can also crop that to 100% and get a very decent shot.
Also; the Tam is a bit of a vacuum cleaner and I rather have her pollute my D7200 than my D810.