Upload & Sell: On
... my success rate (defined here as simply in focus)
That's a fine criterion.
Man, that gym must be dark! According to my matriculations, f/2.8 at ISO 6400 gives you a 1/500 shutter speed for 6 EV, which is equivalent to a "brightly lit" home interior. EV 5 would give 1/250, which is a little slow. Oh wait! My* EV table 'equivalent' for 5 EV is "school auditorium"*; you're in the weeds on this one.
I have both the 50/1.4 and 50/1.2L, but I haven't used them for sports, or other fast action situations. I have used the 85L II (which everybody says stinks for this stuff), and it's great for tracking - as long as it's in focus when you start.
Have you tried using flash for 'under the basket' shooting?
* based on http://www.fredparker.com/ultexp1.htm