And what can priest DO while they walk around at that speed? Certainly not lead a unit. You have to look at more then just a single aspect of a class when talking about it being "overpowered". Priest are mostly a RP class, it tells you that when you choose the class. Tom explicitly warns you that there is less to "do" in game mechanic terms for that class.
Even in advy/inflitrators did lose their current movement system, I doubt it is something that will get implemented. I seem to recall Tom has said something along the lines of he doesn't want the advy game to get too deep, too distracting from the noble game.
Finally the point of infils being able to KILL their target. With the exception of the recent BT invasion, All death in Battlemaster is opt in. Allowing Infiltrators to kill their target, even if it was only a slight chance would violate this choice for the player that is targeted.