In fact, these are advanced clan clothes, not deco.
You can wear the deco on them.

And the above mensioned effects(or option?) are applied when full sets are put in.

-full set
1).hat 2).upper & lower clothes (4+4= 8 pockets)
3).wrist band 4).sheos

The quest of GB12 gives the chance for choosing advanced clan role.
When someone acomplishes the GB12 quest, also he/she can get the FULL SET clothes from elder.

Additionally, choosing advanced clan role doesn't mean the change of clan role.
It's a kind of promotion.

If you are a worrior, you still remain as a worrior after finishing GB 12 quest.
And then 3~4 new skills and advanced mastery can be learned.
(one more, 250 stats limit -> increased 300 limit)