For indoor sports, from figure skating to grade school basketball and volleyball, I was quite happy with my 7D and 70-200 f2.8L II. But, there were times in some gyms where lighting was a bit challenging and I often shot between ISO 2000 and 3200 to keep the shutter speed at 1/500 or better.
For figure skating, moving from a 60D to the 7D greatly improved my keeper rate because the 7D is much better at AI Servo in tracking moving skaters. On occasion, the 8 FPS is a nice benefit, but I don't rely on it as a substitute for timing the shot.
Then, I upgraded to the 5D3 and saw a great improvement in image quality at the higher ISO. With the 7D, I was always using noise reduction in post. With the 5D3, I rarely need it.
The 7D and 5D3 are similar in focusing performance. I shoot single-point expansion and do find that the 5D3 has an edge with its extra focus points. But, the in-focus keeper rate didn't improve like it did moving from the 60D to the 7D.
So, it all depends on the movement of your subjects. If AI Servo performance is a must and your 6D is having trouble tracking, then the 7D might be an improvement. I emphasize "might" because I don't know how the two compare in this regard.
If you're happy with the 6D focus performance with sports, then skip the 7D and get the 70-200 f2.8L II. The Mark II version of this lens is not only sharper, but reportedly focusses quicker than the Mark I version.
Regarding the extra reach of the 7D, it doesn't help indoors for sports. You will be using higher ISO and need to clean up the noise later. You will get cleaner images cropping in post with you 6D.
As for FPS, yes, the 8FPS of the 7D can catch shots that I would otherwise miss. But, more often than not, the first shot in the sequence is the one I want. But then, I'm not shooting professional athletes who would be faster and more challenging when timing some shots. You'll have to make the call regarding the value of this feature. Personally, the high ISO image quality trumps the higher FPS.