Aperture vs ISO for action shots

1)Teleconverters do make images softer. Especially 2x converters.

2) A larger Aperture gives Shollower Depth Of Field(Area in focus). So F2.8 would give better bokeh(Soft background) than F5. For Action shots I think F2.8 is fine.

3) Maybe a F2.8- F4 should be fine, Coupled with ISO between 200-800. Normally ISO should be fine on the d600. With noise reduction, IS0 800 should be ok.