you're so completely wrong... of course players play at nearly the same speed at 99 threshold, because that is the maximum! just like all WR's catch nearly as good at 99 catching, or QB's are all deadly accurate at 99 QB accuracy
however, the difference between 80 and 90 speed guys at for example a 40 threshold is greater than the difference at a 60 threshold...
the threshold will expand the gap between EVERY speed rating the lower it goes... just as it will minimize the gap between EVERY speed rating the higher it goes