Aperture vs ISO for action shots

As I understand it "stopping down" a lens or raising the aperture lets in more light but in some way that creates a softer image - not sure why.  This means to get the same "brightness" in an image you have to either lower the shutter speed or raise the ISO.  For action shots lowering the shutter speed introduces motion blur so I feel raising the ISO is the lesser of 2 evils.  My question is how do you determine which action - lowering the ISO, or raising the aperture - has a greater influence on the final outcome of an image?    Would f2.8 @ ISO 200 be better than F5 @ ISO 800?   Which effect - extra noise form high ISO or softness from lower aperture is easier to correct in post processing?  Below are a few shots which hopefully illustrate my conundrum.

A little blur from slower shutter + a little soft from open aperture

Not blurry but a lot of noise from ISO 800

These were taken with a Nikon D600 + 85mm f1.8g Lens + Kenko DGX Pro 2x TC

I have heard that people get acceptable images with the D600 at ISO 6400 but based on the ISO 800 image here that seems unlikely.  Am I just expecting too much ?  This is my first DSLR, or is the Teleconverter having a significant adverse effect?