canon – Which lens is best for athletics?

So I'm an athletic photographer, and I have a dilemma. First, I use the 5d mark II body. I currently have the 100-400mm f / 4.5-5.6 USM II and I LOVE … except for the interior photos. Before buying this lens, I used the 70-200 mm f / 2.8 for indoor volleyball and the photos were excellent, but I needed 400 mm for other outdoor sports. But recently I went to another indoor game, and the quality with the 400 mm is just terrible. I am considering changing my 400 mm by 200 with a 2x extender, so I can take those photos indoors. But will slow autofocus with the extender be too drastic for outdoor sports such as soccer and horseback riding (I also ride)? Thank you!