It's just like elite / super crap has more effect at higher levels, +'ing has a slight effect on your weapon's atk, an effect which increases slowly with each upgraded level, and additionally with each +
The lower the level, the lower the % increase, to where at a very low level it is non existent.
For example, a super +9 dress is exactly the same as normal.
But, a super +9 lvl.110 archer coat is something like 3-4 maybe 6 times that of normal (not sure, but).
So, the higher your lvl, the bigger the difference.
The very low level +'s are still good to use when composing, but practically worthless in battle.
You can try it yourself, or take my word for it.