During MOSFIRE commissioning, we acquired imaging data on a field as it transited near the keyhole to determine at what rotation rate guiding performance will start to degrade. Below are two graphs that plot the guiding uncertainty vs the elevation and rotator speed.
Above 86 degrees or a rotator speed greater than 2 degrees/min, the guider tracking uncertainties jump. Between 87 and 88 degrees we stopped guiding because the tracking was extremely poor as evidenced by the second jump in tracking performance at 5 degrees/min.
The 86 degree elevation guideline is good to first order, but the limits are dependent on the declination of the object. If for instance, you were tracking a target at a declination that matched that matched the observatory latitude, the telescope would track well all the way to ~89.9 degrees at which point the telescope would have to slew 180 degrees in Azimuth before crossing the meridian.