Canon EF 100-400mm f/4.5-5.6L IS II USM Lens or other options?

So, I know that you are agonizing over the purchase and wondering whether you can shoot wild life or sports.  It's freezing cold here and the light is fading (I'm shooting at ISO 1600) but I ran outside in my backyard to take a couple of quick snapshots to give you some relative ideas.  No, they aren't identical settings and therein lies some differences when you look at the images...

The biggest issue when shooting "long" with the 100-400 is the loss of light.  The largest aperture with a 1.4x is f8.  Contrast that with the 300 f2.8 + 2x III TC f/5.6 (I think this image is f6.3?)  The largest difference between using a faster lens is that one can use a higher Tv, reduce the effects of motion blur, and/or lower Av to produce a better bokeh or effect (though you can do the later in PP).  Now, these were all hand held and don't really demonstrate the potential sharpness of plopping the lenses on a gimbal/tripod but it gives you some idea of what you could do walking around.

300 f/2.8 IS II + 2x iii TC (600 mm) cropped around 50%

100-400 IS II +1.4x iii TC (540 mm) cropped around 50%  Note the f difference!

This last image is simply to illustrate that even with a 1.4x tc, the 100-400 II IS can AF in "tougher" conditions.  Low light, backlit, busy background, etc.  Unbelievably, this was shot hand held at around 1/50 sec.  No, it's not anything that I would print or display and if you want razor sharp images you need a tripod, fast shutter speeds, and good light.  But my Sigma 150-600 could never do this and it would just hunt and sputter...

100-400 IS II +1.4x iii TC (540 mm) cropped around 25% to demonstrate relative effectiveness of IS and tough AF conditions...

Does this help?