if the main problem is priest being able to change sets to fight an expecific class then i dont see how a bigger timer will help... if you are going to fight a class that requires you using equip set X you will probably change it and fight all the fight against him with it, if you find another class that need equip set Y even if the timer is set to 15 sec this 15 sec will probably be over while you were still fighting enemy 1 =P
only thiefs and warriors/fencers seems to really require changing equips more often cause of blue and it seems they will not be overpowered if they cand do this so i think the timer dont make a big difference in other classes...