I started this thread to address the wide end and prefer not to dilute with comments about the long end --- which might be better if you started a separate post addressing the pros and cons of each at the long end. But, I will address some of the above below.
You wrote: The need for a longer lens on FX to achieve the same FOV is to some degree offset by the better noise performance. Adding a 1.4 TC loses you a stop of light, but for exposure purposes you can increase the ISO a stop and get about the same noise performance as the DX at one stop less ISO and sans TC.
The need to increase ISO is only true if your subject is not moving and you are not using a tripod. If not then compensating for the 1-stop light loss of a TC by increasing ISO a stop is valid and means that FX must give just as clean results as DX at one stop higher ISO --- and it certainly does. I suspect that for many, whether you ar eusing FX or Dx, that your lenses are probably maxed out which is why you own them. I frequently am maxed out using my 1.4x TC on my 70-200 and 120-300 f/2.8 with a TC and still want more. For landscape I want to go to 300mm Dx (450 FX) or longer if I am doing landscape extractions or want to include a large fireball in my composition where I use filters and 70-200 + 1.4x TC. For wildlife, most shooters always want more, even if they had a 600mm f/4 on DX
Albert J Valentino Nikonian Moderator Emeritus Vantage Point Images Mastery of Composition is the Key to Great Photography