@johnny_5 if you only have MR56 to work with and we're talking about installing the APs up at the peak of the "A" shape of the ceiling, I'd advise against that for anything over 20 feet as a rule of thumb. As @GIdenJoe mentioned, the best answer is always "site survey" as no two deployments will ever be quite the same.
That may not be what you wanted to hear, the 40 to 50-foot spacing is fine but if the peak is 30+ feet then most/all of the RF energy is going to far above where the clients will be, except perhaps in the rear top-most rows of the bleachers. Will there be signal down at the floor level, sure there will be some but it could be far from optimal and give a poor experience, lower data rates therefore higher airtime utilization, etc.
So your options may boil down to wall-mounting those MR56 APs at a more reasonable height, or leveraging MR46E with semi-directional antennas like the E6 wide patch. Hope that might help!