I am gettying a feeling that in order to detect prox to spawn something you will need first to spawn something to detect proximity =)
Of course just a "watcher" trigger prabobbly will use less CPu/ram than actual mob, but then you running in number of issues to set a whole new way to respawn mobs, encounters, named etc who should be otherwise "UP" and perhaps even trackable (may end up been more complicated that you allready covered)
Another issue- a player running at high end SoW (60%+) speed will be covering a lot of ground VERY fast - this may result in mob spawnign right on top of him as he appraoches since delay in spawn is unavoidable, as well requring you to despawn as player moves away. Now if you got 5 players runing at sow speed in near prox after each other, the whole system turns into spawn fest and i would say MASSIVE server side CPU work as server will be requred to chain spawn/despawn/spawn/etc mobs as group of players runs by.
Not cirticizing idea itself - I just belive its a too complex approach to improve server performance
