Lens Advice - Canon 70-200L+2x, Canon 100-400L or Sigma 150-500

Dear DExperts...

I currently own a 70-200L+2x extender, and I think the images are soft on the 400mm / ISO 6400 (yes, pushing things to the limit but lights at the gyms are very poor). Canon 5D MK III is making a difference.

My question is : is the Canon 100-400L or Sigma 150-500 much better than the 70-200L+2x at the 400mm end ? Here are some samples from the 70-200L+2x as reference. Please share your photos if all possible on the same limits.

Maybe this is as good as it gets. But thanks in advance for any experiences you can share.

Cheers.