I've been super confused by this whole conversation about the cover. I didn't even know High Speed had one! I've played about 1500 games on mine, which came without it, and the ball has never once flown off.
Occasionally on a REALLY hard ramp shot, the diverter will come in late and send the ball up into the glass, but it stays in the ramps and continues on along its normal path. This would almost certainly be fixed by adjusting the diverter to trigger a little sooner. I know it needs that adjustment because a few times the ball has been too fast for the diverter to catch up, so it continues through and back down to the upper right flipper.