i believe you should make them harder to obtain.. not preferrably upgrading composition, just super ones and stuff. if you had normal rates (as i believe you mentioned) talismen wouldn't matter so much. normal rates doesn't mean easy 2900 cps for a +6 stone, including 15 dragonballs to super your talisman, unless you bought one from shopping mall. most definitely decrease rates of them, not so much mob damage taken, most dependant on pvp situations. let me give some examples:
archer (level 100 // w/o fan&tower) vs. trojan (level 130 // w/o fan&tower)
-minor damage is taken.
archer (level 100 // w/ fan&tower [sup+5]) vs. trojan (level 130 // w/o fan&tower)
-damage can be fatal to the character ..30.. levels above the archer, simply due to ONE
item.
now, of course if you recalculated attack/defense of the talismen, major damage couldn't be dealt just because of that one item. if you would rather keep the rates, make the items costlier to upgrade. simple.
dothackking if you don't mind, add me on msn if you would.