Upload & Sell: On
Scott pretty much sums it up. Having both options, I find, one way or the other, that you get past the FL limitation just to run straight into the next limitation which becomes more of a struggle in PP to dig out your image. The breakthrough is partial, and depending on light and lens, your degree of satisfaction may be limited. With FF+TC you will likely have less background separation due to deeper DOF and with crop you are going to run into noise very quickly.
Of course the main question is what do you want to do with the image. If it is just for yourself, it is an interesting experiment and from time to time, between disappointments, you will get something really good. If it is for a specific use, don't count on consistency, especially if your light is less than good.
That all said, I prefer, for example, the 5D3+1.4TCIII+100-400L (560mm) to the 7D+100-400L ("640mm"), mainly because of body performance and noise headroom.
Canon EOS 5D Mark III EF100-400mm f/4.5-5.6L IS USM +1.4x III lens 560mm f/8.0 1/800s 1250 ISO +0.7 EV
Canon EOS 7D EF100-400mm f/4.5-5.6L IS USM lens 400mm f/6.3 1/1000s 250 ISO 0.0 EV