if you made it a percentage, however i could do the quest as easily as them but the reward would remain huge for me and less significant to them.
Isn't that what it's like with all quests that give stat increases though? Take the Shaolin Reborn quest for example, you get a +15 stat increase altogether which is negligable to a high level player but it means a lot more to a lower one.
With a percentage decrease in stats you are at least making the quest the same difficulty for everyone.