In my opinion, you might want to leave around 1mm of play. The just about all materials expand when they get hotter, so you might want to leave enough room for that. As for too much play, this could cause erratic reading between the sensor points. Is it going to make that much of a difference? Well, many racers will spend hundreds of dollars to make their car go 0.5 seconds faster around a track. To those looking for all the juice they can get, shimming will be an option.

Every motor is different, but they all work off the same principle. IMHO, efficiency is the main goal. Slop, in a rotor, will take away some of that efficiency. With all the different motor brands, I have shimmed, they all responded the best when shimmed with about 1mm of slop.