It's Defense percentage (%), not the rating (Numerical Value). Look at the Melee/Ranged/Spell Defense % under the ratings, they should all decrease by 5%.
I usually have to gather about 5500-7000 of a material at a time to fully utilize the crafting booster. I may be able to store it but I'd like for it to not take up so much of my sacks and space. I agree with your suggestion though, it shouldn't hinder the game play in anyway if the duration and cost were reduced in respective ratios.
(I believe that you can stack boosters for an increased duration.)
How about if someone needs an item, it informs the whole party with a message asking if they agree to the "need" from that individual. If at least 2 other members agree (60% of total votes, including the individual), then he will receive the item, if they don't, it'll be considered as a "greed" roll?